Stiverne is a proud fighter and wouldn't take a dive against Wilder. A big payday against Wlad looms anyway, so it makes no financial sense unless he thinks he'll lose to Wilder anyway.
Interesting, but: 1) Even if true, the rest may still happen 2) Did Jose Sulaiman not abolish unifications in his time? 3) What if Stiverne knows that Wladimir is either ducking him in fear of losing, or for some other commercial reason?