So IPFS is, according to you, not defined as its protocol, but instead as its API? So I could build an IPFS implementation that just grabs stuff from my (centralised) web server and say I'm using IPFS? Either nonsense, or useless - the goal here, I thought, was to build a global decentralised filesystem that looks the same from everyone's perspective.

All DHTs suffer from this issue. It's just particularly likely that IPNS's use of a DHT will lead to attacks. Making it not susceptible to this would require a redesign of IPNS's protocol.