It's not moving focus away from Linux/Office365 but it accommodates people that want to have their Gmail accounts in the same client as their Office 365 / Exchange accounts.
Another reason for wanting a client that works with Exchange and Google is simply wanting to use the same app both at work and at home, even if you never cross the streams and access them both from the same instance.
And as others have mentioned, Linux email clients suck. I'm a huge proponent of open-source software, and prefer free clients wherever possible. No one has made a free email client that I can tolerate. They're so bad that I tend to stick to web clients. Hiri's client reminds me of Polymail on MacOS, which is the only non-game, non-cloud application I've paid for in the past decade.
That doesn't seem like a great idea, on many levels.
I bought Mailbird because it looked amazing and had the feature set I wanted. It has done its job extremely well, and I'm very happy with it. But it doesn't have a Linux client; it's only for Windows.
I have six active email addresses spread across multiple providers - Office365, Exchange, Gmail, GSuite, and Zoho. I need a central place to read it all.
I've seen Hiri mentioned before, but dismissed it because it doesn't support Google/IMAP. Once it adds that support, it has my attention, and potentially my dollars.