So if I understand this correctly, this provides a way for example.com to suggest a DoH server that the client can use to resolve example.com's subdomains? I can see it being problematic because it'll bypass my resolver's ad-blocking.

I don't use any Apple software or hardware, but if Firefox starts using it I'll start worrying about it.

Yes, that's right. I don't know whether this is on the agenda for Firefox or not.