So if someone says "I do know German and the translations were all wrong", parent should feel free to say "Actually, I just meant the English grammar of the subtitles, not the accuracy of the subtitles itself" and you'd be just fine with that?

He's just saying that it's a fine video with these subtitles, so thr subtitles were fine.

The accuracy of the translation isn't that important. Who knows, maybe it was a bad video initially that got improved by "wrong" subtitles :-)