Boston robotics is valued at 1.1bn. That's nothing. Musk could easily buy them from Hyundai without even thinking about it.

If they are happy to sell it.

My guess is he probably figured it'd be cheaper to spend the money on development, especially since it'll be a redevelopment from the ground up following different design-engineering principles as new constraints; including designed to be mass produced which BD robots don't seem streamlined for and are arguably overkill for general use (and not needing to handle heavy loads nor do parkour etc).