Mr Andreesen doesn't need anyone's benefit of doubt, because he didn't say what Ms Lorenz claimed. And even if he did, the use of the word "retard" was entirely appropriate in its descriptive context in that discussion. There is no reason to assume anything besides good faith on the part of the lady who actually did use the word.
Taylor Lorenz, on the other hand, was caught in a lie. Not a little lie, mind you, but a loud, public slander. When caught, she responded not with an apology, but rather another lie, some sanctimonious excuses, and then she ran and hid. Ms. Lorenz has given us no reason to assume good faith on her part here.
This is a lax standard that might be okay to hold for a 15-year-old child, but not for an adult Times journalist.
Isn't exactly what you're doing -- which is to say condemning something for what in normal circumstances would be a mild transgression or simple misunderstanding, and drawing wild conclusions about the speaker that don't seem to be present in the original text -- exactly what the problem with "cancel culture" is supposed to be in the first place?
I mean, if you demand that Taylor Lorenz give Marc Andreessen the benefit of the doubt and assume good faith on his part, why are you not willing to grant the same to her?