That is a point, but it's not a point that matters. It's no different then saying "oh, but browser pixels eventually resolve down to physical screen pixels, so it's the same thing."
The problem here isn't whether or not font size eventually at some point in the browser is getting set to pixels -- it's who sets it. Is the browser setting it? Is the user? And is your layout equipped to handle it when it changes?
The problem here isn't whether or not font size eventually at some point in the browser is getting set to pixels -- it's who sets it. Is the browser setting it? Is the user? And is your layout equipped to handle it when it changes?