I don't know about the author, but I'm on Linux and Android and the apps I see on Notion Calendar are for Windows and Mac for desktop and for iOS on phone.
I've tried the web client a bit when it came out but it just didn't really click for me (as in, I didn't see how it would be better than any email client that has an integrated calendar). Also, calendar web clients just don't answer the issue, in my opinion. And regular Notion is slow and clunky in my experience, so I haven't given them the benefit of the doubt on the Calendar part of their tooling. :)