IFF (If and ONLY if) he still has the chops to play. As the President noted, Kaepernick was great his first year, OK the second, and got benched in the third. After 3 years of not playing, there is no reason to believe he will have improved.
Trump knows Kaepernick cannot play anymore.
Which is why the tweet was Troll Level: Ionosphere!
He was successful for one coach, Jim Harbaugh. Was benched by the next, he opted out of his contract.