The slowest iMac Pro has a max clockspeed of 4.2GHz. The fastest has a max clockspeed of 4.5GHz. The max clockspeed of the A12 is 2.49Ghz. What magic allows Apple to get an almost 200% IPC boost? If that kind of performance improvement is actually possible in the general case, then Apple should exit the consumer market and start selling their chips to everyone. They'd make way more money than they do now.
If something in chip design seems too good to be true, it is.
This is what a lot of people including myself are actually expecting, yes. The most interesting part is that the test is quite real world, not some microbenchmark. It runs to-do list app implemented with modern popular frameworks like Angular.
JS doesn't make a good general benchmark and speedometer is a bad benchmark in particular for that kind of test.
The test itself is basically just writing a tiny todoMVC in a dozen different frameworks. Any one of those sub-tests would fit in only a couple MB of cache (for example, the React todo source is around 550 lines in total).
An i7-8650U has a max clockspeed of 4.2GHz, so for short bursts of single-core processing where everything fits in cache, performance can definitely come close to that Xeon (there's a reason why people buy the Xeon instead of the U-series chip though)
JS uses only 32-bit integers and 64-bit floats and executes on a single core (JS has typed arrays, but they certainly aren't going to be used in those kinds of frameworks).
The power of modern x86 architectures is completely bypassed in that kind of test. A huge amount of the size and TDP an x86 core (I'd guess the majority given the width) is tied up in vector units, but those go entirely unused in JS code (and the JS SIMD proposal was completely killed making that unlikely). What are the other 7 to 17 Xeon cores doing during that benchmark? 30-50% of the TDP in these chips goes entirely to the interconnects because the hardest part of scaling chip design stopped being the actual core a long time ago.
If something in chip design seems too good to be true, it is.