I don't think it is generally considered such? Unless it involves putting out fraudulent/false information.
Porsche execs were investigated/tried in Germany for lying about their financial situation in order to trigger a short squeeze on Volkswagon (i believe none were convicted) -- the potential crime was the fraudulently misleading communications, not the short squeeze itself.
Because this seems just like any other short squeeze, the difference is mainly that it was planned in public instead of secretly in private. And that makes it less likely to cross the line into some kind of illegality, not more, right?
"Market manipulation" itself is not necessarily illegal, if you just mean doing things that will effect the market in a way to bring profit to yourself. It's what tons of big traders do all the time, right? It's illegal when it involves fraud, lying, secret collusion, insider trading, dereliction of fiduciary responsibility, etc. Any kind of 'short squeeze' could certainly be considered "market manipulation" in the plain English meaning of those words, but that doesn't mean it's illegal.
This investopedia article [1] on 'short squeeze' says "Contrarian investors may buy stocks with heavy short interest in order to exploit the potential for a short squeeze" -- it does not suggest there is anything illegal about that. Is that not exactly what wsb did, no more no less? they collectively decided to buy shares to exploit the potential for a short squeeze.
Investors do this all the time, the unusual thing here is that it was planned completely in public, and that it involved such a large number of small retail traders (in addition to a few pretty big traders... and quite likely other non-wsb-affiliated big traders we don't even know about who saw what wsb was doing and decided to go along for the ride, or who saw the short squeeze potential themselves independently).
Most traders don't put their "market manipulation" in public because a) they don't think it could work if they did, and b) they know they cross grey areas of legality all the time, so don't want their business to be out in the open like that.
But in this case... what law was violated?