You can see the moment at which there was no further debate as to what to do:

23:45 DBordello What chain is Mt. Gox on 23:45 gavinandresen Eleuthria: if you can cleanly get us back on the 0.7 chain, ACK from here, too 23:45 MagicalTux DBordello: we stopped import before the fork 23:45 Eleuthria alright

https://buildingbitcoin.org/bitcoin-dev/log-2013-03-12.html

Which is further evidence that it is economic nodes and not miners who control Bitcoin. In this case, there was mt. Gox with like 80% (educated guess) of the economic activity of Bitcoin at the time. So a miner (pool) switches to follow the will of the economic nodes (i.e., whichever side Mt. Gox was on), even when that one pool alone had majority hashrate (both sides combined).