Well in fairness to GM, Musk already used Tesla's allotment of rebates, unless they're new ones in the new bill. So Musk's recommending the government drop all rebates would only punish GM and other Tesla competitors.
Not really. The old, original rebate was limited to 200,000 cars per manufacturer. Tesla and GM already sold over 200,000 cars, so they are currently not getting that rebate. Any car manufacturer who is just now starting to sell EVs is still elligible for the same rebate, so it is an even playing field.
The new planned rebate would be a subsidy for all manufacturers - Tesla, GM, Ford, Rivian, Lucid, so all of them would benefit. However, at a time when the EV demand is greater than supply, the subsidy is not needed. If the subsidy goes through, all car manucturers would benefit from it equally and the plan would be footed by all US taxpayers whether they are EV buyers or not.