What evidence is there than industrial customers want general purpose humanoid robots over specialist ones?

There are plenty of positions were you have normal humans doing only a handful of tasks.

Checkout youtube on some chinese factories building like rice cooker and co. They have like 10-50 stops were one person only does like 1-5 things. Putting tape on, screwing something together etc.

I can see it as the last niche were the real big specialised and for purpose build robots are just not economicly

Musk has a very spiky character sheet. He is, in some dimensions, extraordinarily stupid, and I believe his ego makes a lot of big decisions. But something that might fall into the genius category is this: building things speculatively, primarily for the capabilities that you anticipate developing along the way, the nature of which are not yet known. But this increases your odds of having capabilities in the future that others lack, which looks a lot like a venture capital oeuvre.

To condense that, i might use a phrase like "blind-buying future option space"

Whether Musk deserves that credit is a moot point. I haven't trusted a thing he's said for years, and studying him for revealed intent can't get past "clown on drugs" without violating occam's razor.