of the network is supplied with stripped, non-full data the network no longer enforces full validations. they are automatically trated as scond class(downstream/bridged nodes) or as you call it "compatible"

new rules change without the network having to upgrade nodes.
take segwit. old nodes treat segwit as "compatible"... (the buzzword you love)

or as i call it network consensus bypass
yes we both wish for devs to used real consensus. but the bypass ("compatibility" as you call it) allowed it to change without mass node upgrade to 'opt-in'

as for the pools. the november 2016-summer17 election for segwit was consensus. and it only achieved ~35% vote.
as it was a wave a new version number if you want in...

analogy wear your old dirty underwear but wave a clean pair of underpants if you want in. only 35% of pools waved a clean pair of underpants (new vrsion number)
but then the 'mandatory update'(august '17) wasnt an opportunity for pools to wave their old underpants(stay with old version number) in the air to say they are sticking with things as they are. to prevent the new rules

the mandatory update was wear the old underpants, but you better wave new underpants(version number) in the air or your off the network. thus if pools didnt, they would be off the network
EG if there were 20 pools and only 2 pools waved new underpants(new version number) the network would be at 2 pools but show as 100% adoption... dont you get it. fake election.

you even said it yourself that core can write whatever code they lik and not be told what they can and cant do.
luke JR also said it when h said about the compatibility/ "inflight upgrades" bypass
gmax said it with is buzzword of bilateral split

core did not use the consensus election in the intended way. they bypassed it.

thus segwit got activated even without general public nodes having to change. or vote it in to a full community acceptance level. those opposing it just got ignored

then a couple weeks later it only needed one pool to create new skidmark free underwear and all nodes wont be checking that the underpants are really used(signatures of segwit tx's) as it blindly passes the 'are they underpants' test because it bypasses the full validation. (segwit provide old nodes with something that is not full data, that old nodes dont signature validate but just accept as "compatible" even though what they recive is not full real true validation data)

again the devs admit this.
downstream filtered(gmax buzzword)
bridged/stripped (luke Jrs buzzword)

again learn about luke jr's consensus bypass that he calls "inflight upgrades"