Again, no company spends tens of millions of dollars and years of engineers time just to test a single feature. People who keep on repeating this have zero experience in the silicon industry.
Now, projects do get redirected and companies can get caught off guard by the competition. That's when you scrap projects. So if this is Sony's APU, it's possible that Sony got wind that Microsoft was building a more powerful chip and they changed directions after it was already taped out. This is when you can get some valuable testing time in while the design team starts working on a new more powerful chip. But you don't start a project with the intention of just producing a test vehicle. Especially for something as minor as backwards compatibility.
it's too expensive. It takes too long. And it takes engineers off of the product you actually intend to sell. It slows everything down. Time is money.
My theory was that Sony didn't have anything substantial to work on otherwise, hence the "waiting for big Navi" part.
More "evidence" points to PS5 being on big Navi, and with the knowledge PS5 had dev kits out far longer than Xbox, and Xbox only started sending theirs out in mid November, that could point to Sony using currently available AMD tech back in early 2019 for dev purposes, while Xbox waited for big Navi to be available.
The Oberon chip wouldn't be just for BC, it would to get dev kits out early while Sony waits for AMD on big Navi.
AMD not being able to deliver the specs on the timeline Sony wanted is pretty believable to me.