Comes down to this:
There are MMA fighters who don't train Muay Thai.
There are MMA fighters who don't train sport wrestling.
There are MMA fighters won don't train sport boxing.
Most train at least 2 of the above 3. Some train them all. But some train only 1 of the above.
But its hard to find ANY MMA fighter at the pro level who doesn't train jiu-jitsu. It just doesn't happen.
No one enters the cage without having trained in triangles, arm bars, RNC's, guard, etc.
Part of the reason some jiujitsu isn't seen as working as often anymore is because ALL the fighters train jiujitsu. DEFENSE is just as much a part of jiujitsu as offense.
So when a great striker ends up in someone's guard, and he successfully defends chokes and armbars, people always say "See, BJJ doesn't work".
They don't mention that the striker spends 90% of his training time learning jiujitsu's defense techniques.