A false DMCA takedown is equivalent to perjury. If SteamDB were to counter-sue Sega, Sega could be punished for perjury, forced to pay SteamDB's lawyers fees, and potentially lose their copyright on the content they claimed was violated.
Really? If the risks for making a false DMCA takedown request were that high, I would expect to see less false takedown notices, and less aggressive bots. Or maybe the aggressors are just assuming that the victims don't have the resources to fight a megacorp in court? If it's the latter it seems like an organization like EFF should step in to fund such a fight.
However, they are still liable and can absolutely be sued civilly for an improper takedown notice. But the penalty probably wouldn't extend beyond a monetary payment.