You mean this behind the scenes footage? As a boxing instructor, let me point out a few things:
The power of a knockout punch doesn't come from the arm. It comes from the body. Notice how Tony Bellew isn't torquing his legs and he's barely turning his hip. He's flopping his arm around -- which is fine because the camera is tight on Michael B. Jordan's face. This is how they fake it to look like a strong punch.
The "punch" isn't even striking with the knuckles. Bellew's thumb is facing Michael B. Jordan, so it's more accurate to say he's slapping him.
No one actually falls like that after suffering a KO concussion, with their arms neatly tucked into their body and landing carefully on their belly side so their skull doesn't whip back and get fractured. Let me use the Rousey-Holm fight as an example. Note that at 0:41, when Rousey takes a roundhouse to the chin (absolutely a knockout blow), Rousey's right arm stiffens up and sticks out. This is called the fencing response which is an indication that the victim has suffered a concussion.
So no, Michael B. Jordan most definitely did not get knocked out. No Hollywood studio executive I know of is dumb enough to jeopardize a $35 million dollar production on 'authenticity' by allowing their star actor to get knocked the fuck out. This is not Hong Kong, this is Hollywood. Not even Hong Kong is Hong Kong anymore. Their own film industry has gotten much more strict about this sort of this which is why we haven't seen a true successor to Jackie Chan -- the stunts he did are simply way too dangerous for any movie studio to take on today.