First off, it had already been revealed that his Murkrow did not hold Roost (Toxic, Taunt, Substitute, and Tailwind had all been shown). There's also no way to know if his Dragonite hadn't had max speed or not, simply for outspeeding Scarf Terrakion under Tailwind alone (I know I personally would EV it that way, anyway, because that's just way too invaluable), and Terrakion was too pivotal to my chances of winning that match to take a wild stab that I would outspeed him. Just because Venusaur outsped him doesn't mean anything, because it's very possible that I simply won a speed tie. I didn't want to take the chance there.
Also, if he was running a Haxorus with 252 speed (I couldn't have known or not at that point, and he doesn't even need a positive nature), he would have been totally fine without Tailwind, as he would've lived any Terrakion hit and OHKO'd back while outspeeding Darmanitan for the OHKO. There are a ridiculous amount of scenarios that could've played out, I just acted according to the most complicated thinking process possible at that point and it backfired. My opponent then proceeded to capitalize.