And the damage to Enterprise was much more extensive than that of Reliant. They were fully functional again within a few hours after all. Enterprise was still beat up by the time they got back to Starbase in the next movie. If Kirk got shields up a couple seconds sooner, that would've been a very interesting fight. I think "doesn't stand a chance" is a bit overstating it though. Reliant would've still gotten in the first hits to be sure, but Enterprise wouldn't have been crippled, even if at a disadvantage given they'd have lost a good portion of their shield power. It would've been a slugging match to be sure from that point as Enterprise is making up ground with their superior firepower for the rest of the fight. I think Enterprise(especially with Kirk commanding) would've come out ahead, but I suppose we'll never know there.