Data's strength isn't being undersold. We had already agreed that Data would win, it was just a question of time and Cap eventually getting tired.

You should watch the Winter Soldier, though. WS has pretty similar strength feats, and Cap was able to fight him on pretty even terms. The difference here is that WS was also a (somewhat) human being. Or a cyborg, anyway. He also eventually got tired, his performance impaired through a battle just like Cap's did. Data's wont.

Endurance counts.