They’re referring to the quality of integrations with third-party systems, like the built-in CalDAV support basically every OS has. For some people, using just the calendar app is fine, but others want that deeper integration so they don’t have to rely entirely on Proton to provide features in their frontends that OS apps might already handle.

For example, on Android I might want to let other apps access information from my calendar (e.g. my launcher so it can show me events from within its built-in schedule widget). Same goes for my Thunderbird client on Linux, it’d be nice to have the calendar events be integrated there too. Unfortunately, they currently only support a mail bridge, but the official Proton account on Reddit has made a few comments stating that they’re “looking into” adding CalDAV support to Bridge, but there’s no official timeline on when or if that’ll actually happen. I’m willing to bet it eventually will, but I’ll say I’d definitely appreciate it if they did.

EagerEagle posted a good comment under this post going over the client code stuff, pretty enlightening stuff.

Saw someone post that City Journal article on mastodon a couple days ago and I’m amazed that so few people picked up that the City Journal and the article’s author are basically puppets of the Manhattan Institute, a conservative think tank. I know most people aren’t tuned to look out for think tank propaganda but it came off as really obviously FUD-y and unsubstantiated.