> How does this prevent a random application from making an HTTPS request to a random hard-coded IP address? Similarly, how does this prevent an application from making an HTTPS request to a generic host (e.g., api.example.com)?

I think that, franky, even without DoH, that ship has sailed. WhatsApp and Telegram (to name a few) is known to embed IP addresses into their applications. It is silly to assume that not standardizing DoH will not result in the same situation, and I imagine there are custom DNS bootstraping happening, for good or evil.