How did they know that? Chazal's approach to Midrash

(1) Now Korah, son of Izhar son of Kohath son of Levi, betook himself, along with Dathan and Abiram sons of Eliab, and On son of Peleth—descendants of Reuben— (2) to rise up against Moses, together with two hundred and fifty Israelites, chieftains of the community, chosen in the assembly, men of repute. (3) They combined against Moses and Aaron and said to them, “You have gone too far! For all the community are holy, all of them, and the LORD is in their midst. Why then do you raise yourselves above the LORD’s congregation?”

(11) כבד פה SLOW OF SPEECH (lit. heavy of mouth) — I speak heavily (with difficulty); old French balbus. (English = stammerer).

(26) "And the child (Moses) grew" - Twenty four months she nursed him, and you say "and the child grew"?! Rather he grew, not in the way of the [rest] land. “And she brought him to the daughter of Pharaoh…” The daughter of Pharaoh would kiss and hug and adore him (Moses) as if he were her own son, and she would not take him out of the king’s palace. And because [Moses] was so beautiful, everyone yearned to see him. One that would see him would not leave his presence. And it was that Pharaoh would kiss him (Moses) and hug him, and he would remove Pharaoh’s crown and place it on his own head, as he would do to him in the future when he was great. And just as the Holy One Blessed is He said to Hiram (Ezekiel 28:18), “…I have brought fire from within you, it will consume you…”, similarly the daughter of Pharaoh raised the one who would punish her father in the future. And even the anointed king, that is destined to punish Edom, sits among them in their [own] province, as it says (Isaiah 27:10) “…there shall the calf (the Messiah) graze and there lie down and destroy its branches.” And some of the observers sitting among them were the magicians of Egypt that said, “We are wary of this, that he is taking your crown and placing it on his head, that he not be the one we say (i.e. prophesy) that will take the kingship from you.” Some said to kill him, and some said to burn him. And Jethro was sitting among them and said to them, “This child has no intent [to take the throne]. Rather, test him by bringing in a bowl [a piece of] gold and a coal. If he outstretches his hand towards the gold, [surely] he has intent [to take the throne], and you should kill him. And if he outstretches his hand towards the coal, he [surely] does not have intent [to take the throne], and he does not deserve the death penalty.” They immediately brought the bowl before him (Moses), and he outstretched his hand to take the gold, and Gabriel came and pushed his (Moses’) hand, and he grabbed the coal. He then brought his hand along with the coal into his mouth and burned his tongue, and from this was made (Exodus 4:10) “slow of speech and slow of tongue.” “And she called his name ‘Moshe’” – From here you can learn about the merit of those that perform acts of kindness. Even though Moses had many names, the only name that was set throughout the Torah, was the name that Bathyah daughter of Pharaoh called him. Even the Holy One Blessed is He did not call him by another name.