Is there any explanation for why they decided to do this? I would imagine they just don't have the means to test on Mac anymore but I'd like to know why for sure.
I am not sure what the current progress on OpenCL support is, but Tensorflow's configure script definitely asks whether you want to install Tensorflow with OpenCL support.
The last Mac to ship with an NVidia GPU was the 2014 MBP. That was three years ago, and is starting to exit most companies' hardware refresh cycle. You can buy an external thunderbolt box and try putting an nvidia GPU in it, but the driver support isn't always functional. For an example of some of the hoops you have to go through, see: https://9to5mac.com/2017/04/11/hands-on-powering-the-macbook...
And I'm sure you can imagine the reasons a large company wouldn't set up a Hackintosh. :) The writing is on the wall for being able to maintain a reliable test cluster, at least, for now, and broken tests are very very bad for being able to develop rapidly.