What does "watch your arguments" even mean? It's just "Dont say anything I disagree with" with a threatening tone.

Not at all. In this case a completely false argument was made, one which was immediately obvious to me because something that I had positive experience with was referenced in an all encompassing negative light.

Why bother asking a question you already have your preferred straw man answer to?

For many humans "watch your arguments" carries the same tone as "pay attention to those boxes you're standing on".

A reasonable assumption here would be the GP questioning the assumed universal truth of:

  the same way [ all people ]  are disgusted when eating a "powder shake" kind of meal compared to a whole, earthy, flavorful, home-made meal