If 90% of users leave it as shipped set to (say) Google, 9% switch to their preferred service and only a handful like you actually type in a DNS name, that freedom to do is preserved. If it doesn't check the certificate from the DoH server when you override the configuration, you are immediately exposed to a MITM so it needs to check. However you'll have no trouble getting a cert for your own FQDN, and that's what the poster you replied to is thinking of.
I believe there are ideas for a network-level autodiscovery of DoH servers, analogous to regular DNS. That would solve the problem, but I can't see how it would get buy-in from the DoH crowd, as it would tie DNS to the network again - and it would still have to be implemented by every single device.
Of course all of this is moot if the vendor actively wants to prevent you from intercepting DNS.
You'd have to get a certificate for whatever domain the device wants to talk to - a domain you very likely have no control over.
No sane CA will give you a cert for that. The whole point of the PKI is to keep MITMs from pretending to be domains that they actually don't control - which is exactly what we'd be doing here.