Default Health Monitor HTTPS

I think it means that the monitor will basically do a GET request against the pool member IP and accept anything in return to determine it's availability.

You are correct that the default HTTPS health monitor is not particularly useful for testing true application health, since it sends a GET request for whatever the default page is, then will accept ANYTHING as a response, including a 404 Not Found. The HTTPS monitor (along with a lot of other monitors F5 provides) is designed to be customized. You create a new monitor using the HTTPS monitor as the parent, then customize the SEND and RECEIVE string settings (and others) to accurately measure a particular application's health. And you can use different monitors for different applications.