Two facts remain: First, Biden would have been in deep shit with the party if he had chosen a white male. So, in his search for a running mate, he had to immediately eliminate a large percentage of potential veeps. It would be dishonest to pretend that summarily eliminating a large percentage of perfectly viable options -- in any situation, for any kind of choice -- does not hamstring the process and potentially end up with a choice that is based on lower standards.
Second, the proof is in the pudding: I've been paying close attention to how the Democrats are dealing with Biden's biggest weakness by far, the one thing he can't fix or improve, his age. I'm reading the articles that are popping up everywhere about who might be a viable alternative if Biden didn't run. Guess what? I'm not seeing Harris mentioned as a top choice. I'm seeing Whitmer, Newsom, Buttigieg, Klobuchar. Harris is sometimes mentioned, but only with clear doubts.
The party's first option if Biden didn't run should absolutely be his Veep. Easily. Clearly. Obviously. And it's not. To ignore a possible reason why is dishonest. The party has played stupid political games with race and gender for a long time now, and it's biting them in the ass. It's largely what brought us Trumpism.