Musk co-founded X.com along with Harris Fricker, Christopher Payne, and Ed Ho. X.com merged with Confinity in 2000 to form PayPal. So yes, he was a founder unless you take the position that nobody founded PayPal under that scenario.
Discussion
This article suggests he isn’t a founder, but if you merge with similar company isn’t it a 1 + 1 = 2 type of thing? It’s definitely convoluted so who knows? https://tfipost.com/2021/09/unmasking-musk-was-elon-musk-really-a-cofounder-of-paypal/amp/