Thanks for sticking your neck out. When these kinds of articles happen, it often feels like a witch hunt.

How extensively does the Gmail team test interoperability with other mail systems? From my personal experience, one sin committed by many teams who in say they play nice with the ecosystem is to neglect to test integration against other players in the ecosystem. It's annoying to test and slows down development pace, so frequently no developer wants to do it. This leads to a situation where it may be unintentional, but there is definitely a "favored client."