I mean, no one disputes this.
What it isn't was 300 Spartans and a few hundred other Greeks facing a gigantic army of hundreds of thousands of Persians. The Greeks numbered as high as 10,000, and only managed to stop the Persians for a while due to how they were in a narrow pass where Persian numbers actually worked against them.
What is inaccurate is to say that the Athenians and Spartans managed to save Greece from a Persian conquest. In reality, by the end of the Peloponnesian War, all or most of Greece was vassal territory of the Persians. They were in essence part of the Sardis satrapy. Alexander's conquest is more accurately a revolt (especially because Macedonia was actually conquered by Persia before Darius decided to press on the rest of the Greek peninsula), just like how the Manchu conquest has to be more accurately described as a revolt since Manchuria was officially a territory of the Ming.