The Dept. of Justice has a policy against indicting sitting Presidents but not necessarily former Presidents. Trump's lawyers are basically saying that since he was a sitting President during Jan 6, 2021, they shouldn't be able to indict him now that he's not longer a sitting President.
Right now they are spending a lot of time deciding whether Trump, when speaking to the masses was acting as a President or if he was campaigning for his new term as a private citizen, as well as, whether anyone but the actual sitting President at the time can even answer that question.
Trump's lawyers, at this moment, do not necessarily need to prove that the act isn't treason but only that he was the President and was acting as a President with his statements. If that entire line of defense is taken away, it severely weakens Trump's current defense strategy.