I think we should leave it as a [house-rules] question
That might be surprising, because the ruling they made was also definitely an available official optional rule. But here's the thing: they didn't know that when coming up with their rule. In their universe, not apparently having read Tasha's yet (or at least that section of it), what they created was entirely a house rule unsupported by any material, and then they brought that house rule to us to verify it.
More concretely, these are two very different questions:
- “Hey, I came up with this thing. Is this okay?”
- “The official rules say I can do this thing, and I did it. Is this okay?”
And they have very different answers (in the same order):
- “In fact you unknowingly used something that's already an official rule, so it's probably fine.”
- “Yes, you did that thing correctly.”
Both answers might conceivably have come with corrections as to how the rule works or how to use it well, but that correction would come very differently between the two (same order again):
- “While we're at it, the rule also fully supports putting that point into Charisma, so you might want to allow that anyway.”
- “However, you messed up with the Charisma bit. The rule you were using plainly states...”
Last but not least, one of these questions is at high risk of being downvoted for being a trivial, not-that-great kind of question. The other is a great question and pretty much has no issues at all. I'll let you guess which is which.
Overall, I think this means we should run with the original context with which they asked their question: they made a house rule, they're presenting that house rule they made up to us, it's a house-rules question. As part of evaluating their question, the answer is that hey, it's also an official optional rule—but that's the answer, not part of the question.