IIRC a big part of Google’s coding guidelines is also about making it easy for people not heavily invested in a specific language to contribute safely. So not necessarily a project size but rather an organizational concern.

They’d rather see it done the same way it would’ve been in any other similar language than with a language specific feature.

There are also portability concerns in mind given that projects like Chromium have to be easily portable across a vast amount of platforms (this shows with things like long long which is also on the list).

Some of it is historical reasons or portability more than anything else. Chrome is an old C++ project and evolved many of its own versions of functionality before standardization; and there's benefit to staying on its own implementations rather than switching.