Not necessary. They only need to ship one model of Mac Mini or any small desktop lineup with Ryzen to improve their negotiating position while still being compatible with the existing x86_64 ecosystem.
So no, this is not merely a negotiation jibe. There are definite long-term prospects.
We're already moving to a mobile-first design and approach world. Having those mobile/tablet apps expand automatically to desktop is the next logical conclusion. As an app developer, there is nothing better than write-once-run-everywhere, and the cascading effects of that on the whole Apple ecosystem and future consumer audience is hard to understate. Keeping aside power, efficiency, device prices, opportunity costs and much more.
> significant work by third parties outside Apple who haven't even heard of the possibility of this happening until today.
Definitely not today. Its been speculated for many years now ever since the A4 chips, and its still not official news. Usual "people who don't want to be identified". If or when this becomes official news, most partners would be like about time, because everyone is in one way or another working on convergence and multi-device. Adobe and Microsoft are examples of pivoting many of their desktop businesses successfully to cloud and apps already.