he was likely using a setup like this: You can use Mentats + Suit-type Clothing + "Sharp" legendary armor pieceds to buff up your Charisma to significantly higher levels, of which the game does take into consideration for in game calculations such as Settler Settlement Cap, Prices on goods, etc. it can get your charisma up to 30+ using that setup
According to the wiki here, using everything theoretically possible in the game & dlc, it's possible to reach a charisma of 48, but in terms of benefitting barter values with the game's vanilla vendors there's a charisma ceiling of 16.
If I remember right, I had a legendary crafting mod running that made me hit the cap in the endgame, where I would use this mod to passively give me some caps for settlement building.
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it can get your charisma up to 30+ using that setup