through the power of friendship or something
My guess is that he lost all is abilities and somehow his memories of those abilitys.
It was by chance, before the change of their species. MC's kingdom didn't do well at all, they suffered the same way as all of the other celestial kingdoms. Eventually, he found his answer via an item he discovered within the tower.
Since MC loves his vassals and the duke, he didn't want them to suffer anymore, so he used the potion to make them stronger but with the cost of basically making them turn on him. As we saw when MC took the potion, it was super painful and almost killed him without intervention.
So stuff happened yada yada, MC discovers that his vassals and Duke stats became very strong. So they found the tower and all the stuff previously hard to suddenly easy. But-- when combined with MC's support skills where his vassals and duke get a stat boost based on how much MC trusts them and a healing skill based on how much MC loves them, his vassals and Duke were able to take on the Tower without fear and allowed them to steam roll through it. Remember, MC during this is stilll the same race previously, it was his skills and vassals/duke's newly found power that allowed them to do this.
In some way, like how those tower climbing and isekai manwhas, MC's cheat is the race corruption/changing potion. It basically turned things around for him.
I repeat, MC by himself is not actually very strong but the true brute force of his kingdom are his vassals and the duke, MC is a PURE support type character. Even with the way he handled his kingom before being isekaid, he was more akin to a puppet king who has a high charisma and the skills for recruitment skillful vassals and he left all of the decision making, political stuff and war related things to the Duke.
so MC is like rank 1 overall right and his gap above everyone else is so high it’s impossible to rank higher than him despite him quitting the game before BUT HOW? MC keeps being saved and doesn’t show a single ounce of power or intellect like?? how did he finish all those floors that others cant? i get that he has close bonds with his people specially the rlly powerful ML but that cant just be it righg T_T i need MC to put people in their place on his own istg