... in a very different competitive environment, and with probably a larger number of failures to get to market (although it would be interesting to put numbers to that, if they can be found).

If anything the additional stacks means both app quality and the platform-specific knowledge a senior dev needs has declined, since today you're probably working on a pile of React Native bullshit or something that has very little to do with Android anyway.

True in the case of React Native bullshit, false in the case of a native app that isn't just CRUD. A senior without android experience could jump on the apps that are glorified websites and be productive relatively quickly.

A senior without android experience is going to have to learn the platform; this will take longer than learning the language. The pace of change updates the best practices often enough to serve as a kind of catch-up mechanic but native mobile does in fact require domain knowledge. I would be comfortable putting a senior iOS dev on native android but certainly not as the lead.