Gmail absolutely hits that sweet spot of API capabilities and where the users are, so I can’t fault the project creators (or most every email client business these days) for building first (or exclusively) for it.
That said, seeing Outlook as coming soon on their login page is reassuring that they’re building in a way that won’t tie them to Gmail forever. And while few email providers outside of Fastmail are offering JMAP support, as an API it’s much closer to the degree of functionality expected by anyone building on top of Gmail’s API today. A great new client that gives a big section of the public a better way to “do email” might be what it takes for more services to start offering JMAP.
So hats off to y’all and fingers crossed on incorporating open standards.