Their title on HN and website is "A native macOS email client for Gmail," and that's exactly what it is, especially if you were trying to be technical.
It's like bickering that it's not a native client, it's a macOS client, because it only runs on macOS. Or that you'd feel swindled if it didn't run on Gentoo because anything that calls itself "software" when it's really "Macware" is basically false advertising. It's an odd attempt at pedantry.
"Email client" used to mean something before everyone started assuming Email = Gmail.
Imagine someone submitting a link posted as "A web-browser for facebook.com".
Nobody here would ever accept that a piece of software which could only work on facebook.com to be an actual web-browser.
So why should we allow for the same exception when it comes to email?
I would encourage them to reconsider their "email client" language. It's asking for a headache.