It really depends on if Musk is acting as an officer of the company or on his own. If it's on his own, he's liable in the same way that any individual is liable, and only personal assets would be considered. If he's acting as an officer, then the company he's representing is liable.

At no point would other companies he runs be liable, unless he's also acting as an officer of those other orgs at the time. So they should never consider revenue from other firms, but merely revenue from his personal holdings.

That said, this is coming from a US perspective, I'm not familiar with EU law.