They are just as shady as grammarly.
Years ago they have decommissioned their open source browser extension, made a new shiny one that is closed source, and when people asked why isn't the new one open source, they just closed the issue without any explanation.
Their selfhosted offering also doesn't worth much: the "magic sauce" is not included. But more importantly, now that I tried to look this up again, the only mention I have found about the selfhosted version on their website is a "contact us for the on-premise version" button on the "For Businesses" page, where you can write an email and they will probably respond with a price offering.
For selfhosting it, you may be better off with this fork as it enables premium features, but do note that this is not developed actively. Look at the "commits behind" counter and compare it to the "commits" counter below it.