Long story short, one of these two guys was ousted and a lawsuit settlement in 2009 determined that there were five co-founders, including Musk.
Note that the lawsuit didn't "determine", it was settled out of court, so there was no determination of fact.
Instead the parties agreed to let Musk call himself a founder for undisclosed terms. Musk basically paid for people to let him use that title as part of the settlement.
Just going to leave this here and y'all can do the reading: https://www.cnet.com/culture/tesla-motors-founders-now-there-are-five/
Long story short, one of these two guys was ousted and a lawsuit settlement in 2009 determined that there were five co-founders, including Musk.
Note that the lawsuit didn't "determine", it was settled out of court, so there was no determination of fact.
Instead the parties agreed to let Musk call himself a founder for undisclosed terms. Musk basically paid for people to let him use that title as part of the settlement.