There can be dozens of strains conspiring to form a single fruiting body, without these strains combining into a single organism. This is well known and established in mycology. Paul Stamets reports isolating up to 200 organisms from a single fruit. It's not science fiction, it's science. Picture thousands of individual soldiers in an army going to battle. Each is separate, yet conspire to form the coherent fighting force.
An isolate can either come from a pairing of two compatible hyphae from separate spores, or from mycelium that has later combined via anastomosis into a single organism. There can be monokaryotic pairings with other mononucleate mycelium, dikaryotic pairings with other multinucleate mycelium, or mono-dikaryotic pairings. You know you have an isolate when there is no more sectoring on agar.