Yes, you're right that desktop and 'mobile first' apps are a different category, so that at first, it would seem like the comparison between Skype and WhatsApp is a category error.

But I suppose it's also fair to say that Signal is now a good example of a 'privacy first' app, a new category that WhatsApp equally don't seem too interested in.

I think the analogy still holds both ways: exact uninstalls are not important once the communication starts moving elsewhere, and 'mobile first' and 'privacy first' are categories that companies ignore at the cost of losing millions of users.