I would say, it’s them caring about the product and their needs, rather than the underlying stack.

For first few seconds, I deadass though they are talking about Germans with a height of 1,5 meters.

Quad9 does also offer DoT and DoH.

I can recommend you ICSx5 for ical subscription and DAVx5 for calendar synchronisation.

Yet I would agree, that a good calendar app should ship with these features.

The German technology blog Kuketz has a comprehensive overview and comparison of all major messenger services.