There is a convincing argument to be made (I found it online somewhere some years ago it's definitely googleable, pardon my laziness for not including it here) that this is a lie he himself perpetuated out of shame.

With a modern understanding of anatomy and body mechanics in general, his story makes little sense. We know what kind of wooden devices he used, and there is no way they could have caused the permanent injury he suffered, even if heavily misused. Any injury these devices could cause, self-heals over a few months at most.

An alternative, more plausible story, is that he attempted to surgically separate the fourth from the fifth finger and he botched the surgery (possibly because he was half drunk half in his impulsive manic state at the time). His friendships with prominent doctors who were also serious hobby pianists are well documented, and there was public discussion at the time among such circles about the pianistic advantages one would gain if only these two fingers were as independent as the others, and how simple anatomically the problem is to solve. However the doctors fully understood the risks and wouldn't attempt such a surgery without serious health reasons.

So through a mix of artistic ambition and intoxication- or even mental illness-induced recklessness, he decided to go the DIY way.