First off, it was the opposite of insider trading. They did not have insider information, nor any knowledge that the public did not have about the value of the stock. In fact, because of the ignorance, they had less information than the general public had, which is why they took the wrong side in the trade.
For the most part the "smart money", like Plotkin, were not market makers. They took only one side of the trade, unlike a market maker who would be willing to take both sides. Calling them market makers is false, and more importantly misleading.
Trading of the stock was never suspended on the exchanges. Robinhood suspended buying long positions on GameStop, but you could easily use Schwab, TD Ameritrade, or a thousand other brokerages to trade on the exchange.
The best you could claim was that it was market manipulation. It is a bit of a stretch, but you could claim that "smart money" refusing to pay Robinhood to sell long positions manipulated the market.