He very well may be. He doesn’t know he’s an old man, and the same instincts he had as a younger betta are likely still there. Just like any other betta, he may get along with tankmates, or he may not.
My concern would be more for his health and safety. As an older fish he is probably a little weaker, and his immune system may not be what it used to be. The danger of putting a betta with other fish is only partially about him attacking the other fish. There is also a chance they could bully him or pick at his fins. For any betta that would be incredibly stressful, but for an older fish it could be fatal.
Why not give him his own beautiful 5-gallon tank and let him live out his final days in peace? Curmudgeonly old betta fish probably don’t appreciate annoying young guppies zipping around them. Less stress for him, and he might even live longer.