A mistake or doing something foolish is going out in shorts in the winter and then getting a cold.
He spent days/weeks/months on setting up this deal, he became obsessed with it at some point.
(Hopefully) no one is "blaming" it on the bipolar. It's simply something consistent with bipolar. (But it's also consistent with rich people getting obsessed with something and then spending way too much resources on it.)
> reinforces the narrative
That narrative is simply bad and pushed by people who have no idea what they are talking about. Musk's previous instances of risk taking (that "all" turned out to be fine) is also perfectly consistent with bipolar. (Of course if you look at it then it turns out that he took many risks that made very little sense, like when he talked about taking Tesla private, the infamous funding secured tweet.)
And of course just a label like bipolar means (almost) nothing, these spectrum disorders are complex (and there are usually other comorbidities at the same time, just with different severity), we don't know how he is treating/medicating/managing it, or is it even the right diagnosis or not. Or he could have simply lied.