I think the literal translation would clear things up here.
"Placetne vobis hoc garum?", would translate to "Does this fish sauce pleases you?".
Here Garum is the subject and placetne means pleases. But pleasing to whom? YOU. We don't write it this way for an ease but the meaning would be incomplete if you don't put 'vobis'.
Not really. The translation to "fish sauce" refers more to the modern day fish sauce found in Thai and Vietnamese cuisine. They are essentially identical to ancient Garum (which fell out of fashion in European cuisines over the millennia)
So it means fish sauce in that sense, not in the sense that it is a sauce for fish.