I think the argument is that Airbnb and similar take those apartments from the long-term rental market and put them on the (very) short-term rental market, aka the hotels'.

The problem put forward by those arguing against Airbnb is that this is a zero-sum game. If you have tourists who are able and willing to pay more for a flat (aka "middle or higher class"), this reduces housing availability for "the lower classes".

You could, of course, argue that this would be true even without Airbnb if enough "higher class" people wanted to live in a given city all of a sudden. I suppose that's why people also tend to be against gentrification.