Google's design language is pretty unsurpassed, well documented and very user-friendly, thanks also to a ton of research that has been done in connection with Android, I guess.
So I see nothing wrong with him using that, in fact it would be a major factor for me personally in considering any Google service alternatives.
Keeping the switching barrier low is key for successful and widespread user adoption.
I actually think bad UI design is THE single greatest impediment for widespread adoption of free or open-source software imho. A lot has been written on that subject but I think the trope is "open-source developers cannot create good UIs". It is an old but well-accepted axiom, same as "Poland cannot into space", and it seems everyone adheres to it... ;-)
So using an established and "known good" design language is in fact ingenious.
Joking aside, it's because my friend who help me for the graphic design don't have so much time, and me the developer, don't have much graphic design skills.
Material design is one of the only design system freely available on the internet, with very good documentation, and very good libraries in all languages.
This[0] is an CSS implementation under the MIT license I'm using on 3 different websites[1][2][3] right now. Google can't really take back Material Design at this point (IANAL), which I'd call a feature!
Ignoring that, this stuff all looks fantastic.