Of course Geno is a true Third-Party, he is owned by another company.
The fact that he was in a Mario game doesn't change the fact that another company owns the rights to him, and Nintendo would have to license the IP of the character from Square Enix in order for him to be in Brawl..and what would that make him? That would make him Third-Party
Regardless if Isaac is second or first party, that doesn't change that Geno had more mentions than Megaman for a Third-Party character addition, DeDeDe having one more than him (gasp, is DeDeDe in the game? Why, yes he is), with Ike and Diddy Kong having the same number of mentions as him (gasp again, Ike and Diddy are both in the game!)
The fact that Inafune said that Megaman is not in Brawl as far as he is aware should be reason enough for you to realize that the 3rd party character that got more mentions than him is the one Sakurai chose to put in the game; Geno.