The ARM deal would have impacted all other ARM licensees a lot more than Apple. As you note, Apple is making custom cores so they don't need ARM's reference designs.
For example, Google's Tensor processor in the Pixel 6 uses ARM Cortex-X1, Cortex-A76, and Cortex-A55 cores and ARM's Mali GPU. They add some ML cores to the design, but the CPU/GPU is really designed by ARM. Apple, on the other hand, makes its own internally designed cores for its processors rather than using ARM's designs.
While you're right that the general public doesn't know ARM/Apple's deal, we do know that ARM offers a perpetual ISA license. Even if Nvidia bought ARM, Apple could still make current ISA processors forever (it seems unrealistic to think they don't have a perpetual license). While Nvidia might not want to help Apple, it would be in Nvidia's interest to offer new ISAs to Apple at reasonable rates because a) one probably doesn't need updates to the ARM ISA at this point b) Apple not getting on board with a new ISA could impact the rest of the industry getting on board with it given that Apple is so large (and respected) c) Apple puts a lot of time and money into LLVM and having them against an update to the ARM ISA would (at the very least) mean that there wasn't free labor (from Nvidia's perspective) adding compiler support for the ISA update.
It's true that we don't know all the details about deals between Apple and ARM, but at this point it seems like Apple doesn't really need ARM. Samsung, Google, Amazon, and others use ARM's reference designs. If ARM disappeared, they wouldn't get updated cores and would have to build up in-house design teams. If ARM disappeared, Apple would just keep on making new designs. I think Qualcomm is looking to go more custom in the future as they bought Nuvia and are looking to make inroads into things like laptops over the next few years.
In some ways, it seems like ARM getting bought by Nvidia would be good for Apple. If Nvidia becomes really harsh for third-party licensees, it could mean a few years where the costs skyrocket in the Android ecosystem while their costs remain the same. Even after that, it might lower the number of manufacturers for ARM processors. Would MediaTek spin up a custom-design shop? Would Samsung? Maybe, but it would add a lot of cost over re-using reference designs.
Apple isn't really reliant on ARM for anything at this point. The rest of the industry is pretty reliant on ARM's reference designs. If Nvidia ownership meant that those reference designs went up in cost or if Nvidia wanted its best work to go into Nvidia processors and put out weak updates, that would benefit Apple with Android manufacturers scrambling to figure out what to do: buy expensive Nvidia processors, ship weaker updates, invest in the custom-core route?
For example, Google's Tensor processor in the Pixel 6 uses ARM Cortex-X1, Cortex-A76, and Cortex-A55 cores and ARM's Mali GPU. They add some ML cores to the design, but the CPU/GPU is really designed by ARM. Apple, on the other hand, makes its own internally designed cores for its processors rather than using ARM's designs.
While you're right that the general public doesn't know ARM/Apple's deal, we do know that ARM offers a perpetual ISA license. Even if Nvidia bought ARM, Apple could still make current ISA processors forever (it seems unrealistic to think they don't have a perpetual license). While Nvidia might not want to help Apple, it would be in Nvidia's interest to offer new ISAs to Apple at reasonable rates because a) one probably doesn't need updates to the ARM ISA at this point b) Apple not getting on board with a new ISA could impact the rest of the industry getting on board with it given that Apple is so large (and respected) c) Apple puts a lot of time and money into LLVM and having them against an update to the ARM ISA would (at the very least) mean that there wasn't free labor (from Nvidia's perspective) adding compiler support for the ISA update.
It's true that we don't know all the details about deals between Apple and ARM, but at this point it seems like Apple doesn't really need ARM. Samsung, Google, Amazon, and others use ARM's reference designs. If ARM disappeared, they wouldn't get updated cores and would have to build up in-house design teams. If ARM disappeared, Apple would just keep on making new designs. I think Qualcomm is looking to go more custom in the future as they bought Nuvia and are looking to make inroads into things like laptops over the next few years.
In some ways, it seems like ARM getting bought by Nvidia would be good for Apple. If Nvidia becomes really harsh for third-party licensees, it could mean a few years where the costs skyrocket in the Android ecosystem while their costs remain the same. Even after that, it might lower the number of manufacturers for ARM processors. Would MediaTek spin up a custom-design shop? Would Samsung? Maybe, but it would add a lot of cost over re-using reference designs.
Apple isn't really reliant on ARM for anything at this point. The rest of the industry is pretty reliant on ARM's reference designs. If Nvidia ownership meant that those reference designs went up in cost or if Nvidia wanted its best work to go into Nvidia processors and put out weak updates, that would benefit Apple with Android manufacturers scrambling to figure out what to do: buy expensive Nvidia processors, ship weaker updates, invest in the custom-core route?