Earlier quoted context omitted.
What don't you buy? They test with a certain set of supported platforms. If a client isn't one of those (whitelisted) platforms, then they may send out a bunch of inefficient polyfills so they know XYZ functionality exists, even if it's slow.
Except that it works just fine if those 'unsupported' platforms are treated in the exact same way as the supported ones. To make it seem as if a whole pile of work was done in order to make unsupported platforms work less well but this is a good thing is something that I find very hard to reconcile. And if this were a company that did not have a history of exactly such tricks it might be more believable, MS - with me…
It is _less_ effort to say "We support these specific platforms, and rather than doing actual feature detection, we'll just load a bunch of polyfills to make it work on anything else".