Tempting as the hypocrisy charge is to lay on Republicans, because they are assholes and hypocrites, ad hominem arguments are always logical fallacies, and so they don't really prove anything useful.
The right defense of Biden's promise is for him to say that he had already identified the women for his short list before he made the promise, and that he was going to nominate one of them when the opportunity arose. Suppose Biden had said while campaigning "After a lot of thought, study, consultation, I know whom I plan to nominate, and it's a Black woman!" Would that have been objectionable? He couldn't do that, because the media would have been all over the search for breadcrumbs to figure out who the mystery woman was. So the campaign rhetoric was more vague. But it is inconceivable that he didn't know who was out there.
Language nerds will recognize the distinction between statements made de dicto and statements made de re. A person speaking de dicto does not have a particular person in mind but can describe the criteria. When Petrucchio says that he will the richest woman in Padua, he does not know who she is. But when Tex Beneke sings that he's going home to see the prettiest girl in Kalamazoo, he has a certain person in mind. Petrucchio speaks de dicto, Beneke de re.
Politicians don't make such subtle distinctions. Indeed, they intentionally confound them. Biden could have been speaking de re, i..e, about one or more candidates he had already identified, but he is being accused of speaking de dicto, thereby disqualifying as unworthy every lawyer in the country who is not a Black woman. I don't expect Biden to lecture the country on the niceties of intensional operators. But I do think it would be ok for him to say that he had certain people in mind when he made his promise, and he was simply tipping his had, so to speak, not describing a hurdle only a few people could jump.
Because there's hardly any point in pointing out that Republicans don't mean what they say. Everyone capable of knowing so already does.