The Caps have played a *ton* lately. They've been doing two games a week for several weeks in a row, with a fair bit of travel. This game actually came at a pretty inconvenient time for them, right when the Leagues Cup gets underway for them on Tuesday too. (Where they get to play the Washington Generals again, this time to LAFC!)
I wouldn't have expected them to take it very seriously, and like I say I'm kind of surprised Brian White saw the pitch at all.
All that said, MLS is often thought to be somewhere between Championship and League One levels when measured against the UK (basically the big-money teams in the former, and the others in the latter), so I wouldn't be surprised if League One teams could beat a full-strength Whitecaps team with some regularity. It remains to be seen if Wrexham is on par with League One or not, though, since they only just got there.