As some others here have pointed out, this blog post somehow ignores the fact that some Linux desktop apps have supported online calendars from all the sources they mention (CalDAV, Google calendars, NextCloud, webcal, and probably Exchange/Outlook) for many years, such as the Evolution app (and any frontend app using Evolution's backend, such as GNOME Calendar and probably some others). GNOME Online Accounts also exposes at least some of those account types. KDE's "Kalendar" supports all of these types of online calendars too.
As the author laments, Mac users have no shortage of great desktop calendaring apps (and if you're not happy with Apple Calendar already, you have to _pay_ for a fancier third-party app? Oh no, the humanity!)
On the Windows side, from what I've seen there's also a pretty good "Windows Calendar" app that comes bundled with Windows 10/11 (I'm sure there are other apps out there too, but anyway those who really "live in their calendars" on Windows are most likely already spending their days in Outlook and the whole MS enterprise workflow anyway). I'd guess that the exceedingly few people out there doing calendaring across different desktop platforms are doing just fine with their web calendar or Thunderbird.
Ok, so what is the problem again? If the remaining core of their argument is that there is no perfect open source app "better than Thunderbird's calendar" on all three major desktop platforms, that's a pretty unreasonably high bar and burden to set for open source developers.
The author's rant then morphs into a pitch for writing a whole new calendaring app with a shiny new framework instead of improving one of the existing apps... and in the end, they include this mention in the concluding paragraph:
"if the spirit of benevolence (or frustration) moves you enough, send a grant my way — I’ll take on the herculean task of writing this mythical software myself."
I see the author has a Ph.D, so seeking grants and looking for "perfect solutions" is possibly pretty deeply ingrained into their way of thinking... but the world does not need yet another incomplete project that will fizzle out shortly after the author realizes the sheer magnitude of the problem space and burns out (or runs out of grant money).
What we need is contributors to existing projects, not NIH lone wolves with an "architecture astronaut" mindset.