There is nothing stopping all the miners from simply dropping the btc1 fork code and just using core after segwit is activated, and not all pools actually signed the agreement.
Pools were not the only ones signing the NYA agreement. Hard to believe every NYA signer going to toss his reputation by not honouring the agreement. And we are talking about the most important Bitcoin companies here...
https://medium.com/@DCGco/bitcoin-scaling-agreement-at-consensus-2017-133521fe9a77Most bigger pools/companies use custom software, so the core vs btc1 code is not relevant here, what only matter is being compatible with Bitcoin, like accept up to 2M base blocks in about three months.