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Is this a scheme from adcompanies like Google to circumvent Pi-hole?

Maybe not. Dns over http makes it easy to use for example http://nextdns.io which is nice alternative for Pi-hole (and more convenient IMHO).

I think this is mostly against ISPs who would like to mess with/redirect your traffic (for example show their own page for mistyped URLs).

Pi-hole can just support DoH as well, this doesn't really change anything in terms of adblocking.