Yes, you are right in strict definition that they won't know specifically which app you used to visit a site. Yes you visited for example Hacker News (according to the IP log) but they wouldn't know which of the many Hacker News apps you used to visit it. That is right and accurate, so yes they don't know which specific app you used.

However, the use of the term app is probably more to do with vast usage of apps to now consume the internet. So if you now just use the term "website", many of the general populace (who aren't technical) will think that's fine they won't know because I use apps for Facebook etc.

It's important people know that this covers access through apps. So they simplify the usage of that term.

Technically incorrect yes, but probably a proper description for general readers.

I would assume it's something like the difference between visiting "facebook.com" and "api.facebook.com"