My thinking was along the same line as yours, but the way apple framed it seems to suggest that the M1 accelerates model training and not just inference. Here's the actual quote "It also makes Mac Mini a great machine for developers, scientist and engineers. Utilizing deep learning technologies like tensorflow ... which are now accelerated by M1".
It should be pretty straightforward to test this though: installing tensorflow-gpu on a mac mini and seeing the result.
I suspect, TF's latest branch should also indicate which GPUs are supported.
Curious to hear more thoughts on this.