Agreed, but I think it goes generally to the question of 'character'.
Nothing will change the factual accomplishments\failures, as you point out.
And mythologizing our heroes through anecdotal mis-renderings is a long standing American tradition (RE Lee, anyone?).
But (like with Lee), many of those stories about character are built on a factual basis that, if we can parse it, does gives us insight into the 'character' of the person (in the context of that point in his life, and that time in history).
It is to me noteworthy, from what we factually know, that Grant worked beside and freed a slave when he could have (and needed to ) financially benefitted from doing otherwise.
Whatever else we attach or derive from the surrounding details and (typical) embellishments, this says something about Grant's character, not definitive in itself, but coupled with other known examples and incidents, help us understand something about him.
Few men match the marble of their statues. Some, in their flawed humanity, surpass it.