Earlier quoted context omitted.
I think "disable DoH" there means "block all DoH servers at the router", not "configure Firefox to not try to use DoH".
Blocking DoH at the router is almost impossible because it looks exactly the same as other HTTPS traffic. The only option is to do a man in the middle like some business firewalls (eg zscaler) do. I don't like DoH much either because apps can use it to bypass my local pihole. But blocking it is hard when an app hardcodes its own DoH server. DoH is really a solution for a US problem that we in Europe don't have. Here…
I covered that in the comment next to this one.