He's probably not wrong, but it's hard to prove his side of this. At this point, he's a good back-up and a decent stop-gap QB.
Since he's more of a dual-threat guy, it limits the teams that would be interested in him. It would be difficult if he were a back-up to a pocket passer because his strengths in case that guy goes down are completely different. So that narrows the teams.
Then when you look at those teams, you have to decide whether he's that much better than the next option.
And on top of all of that, teams are likely to deal with some sort of backlash politically.
I doubt you can prove collusion here.
Owners/GMs not signing him isn't collusion. Collusion is 2+ teams getting together and agreeing to both not sign him - not because they came up with it on their own, but because of reason X. Go and read the SI article, specifically points 1 & 2.
I honestly doubt that multiple teams, or a team and the league got together or discussed him to the point that a team decided not to sign him based on whatever reason they were fed by the other party. Again, a team choosing to go in a different direction (even signing a lesser player) because they're cheaper or because they're less of a distraction is not collusion - even if multiple teams do the same thing.
https://www.si.com/nfl/2017/10/15/colin-kaepernick-collusion-lawsuit-against-nfl