Except the week before they'd handed out a Tome, which RAW allows stats to increase beyond 20.

At best the situation is a miscommunication but a good GM would have stopped to ask the player what it was they were looking for.

They didn't. They just pulled an intensely passive aggressive "20.1". That's either a GM flex or a childish fit of pique, neither of which belong at the DnD table.

That's especially true for something as petty as a +1 bonus on a primary stat that's already at +5. At that level, by the math, it's almost all fluff and completely overwhelmed by class mechanics and magic items.

The OP is well within their rights to be angry because I would be too.