While it's nice to see EU hitting where it hurts, and $1bn is a lot of money by most scales, it isn't, insanely, that much to Musk. If I believe Google, his net worth is still over $300bn, so such fine would be less than 0.5% of his net worth.
Given his pattern of behaviour, he may well try to turn it all into a joke and be emboldened for more.
That said, I don't know how much cash Twitter and SpaceX have. Possibly not a lot. Tesla does have more, but it's publicly owned and much harder for Musk to raid for cash.