If your browser's user agent lies outside the "supported" list, you will be presented with a page telling you that your browser is insecure. It's a whitelist. I... Am unsure why you think this is not the case.

You can test it. Or look at any of the many articles after Google made the change.

I set my UA to "Bob Dobbs 42.69". It still loads, and it doesn't say "your browser may not be secure" which is what this thread is actually about.

A freshly installed Firefox, with a useragent of "Bob Dobbs 42.69" yields the "Your browser may not be secure" page, for myself. Both within and without a VM.

So, yes, I think I'm speaking to what the thread is actually about.