Dude would need to summon an ancient golem to resist that divine beast. But nobody has got enough mana to allow him to summon such a thing.
she says "we didnt mean to hurt you..." but lets the little wizard call him incompetent and talk down to him the entire time while also never believing a word he says like how much filpflopping is gonna go on.
The dude doesn't want to be known at all (zero fame), but if he still gets people to know him, he only wants them to consider him an unremarkable, talentless person. Having the little girl mage call him incompetent thus only serves his own purposes. Now the girl knows he can summon an advanced golem, but as long as she believes he can never do it on his own (no mana of his own) and he's even utterly horrible at handling externally injected mana, she will still think he's more or less hopeless. Which is exactly what he wants. If they give him the 10,000 gold he desperately needs and also believe he's a waste of a summoner, they aren't really hurting him.
also kinda odd that the princess hero from previous chapters says she will keep the secret but tells the secret with no regard to MC could have given him a letter or something so he isnt so confused pointlessly the whole time with why he is involved even if knight lady is the teacher
She's still a princess responsible for the kingdom. If the whole kingdom is potentially threatened, she will spill any secret to avoid the threat. What is betraying the trust of one person compared to all the lives in the kingdom? They haven't even met for many years, so any personal feelings would have grown pretty cool already.