It may sound so but the existence of mycorrhiza has been well-proved.
BRBR

I am well aware of the existence of mycorrhiza. But I fail to see any
connection between mycorrhiza and whether certain plants do better in groups.
Also I don't see any connection between mycorrhiza and allelopathy.

Well, orchids don't start growing unless mycorrhiza are present (or a
nutrient solution mimicking them). BRBR

Perfectly true. But that has nothing to do with whether an orchid does better
in a group planting or as a single plant.

You can even assign orchids to taxonomic groups depending on the mycorrhiza
they favor.

Where on earth did you get that idea?
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)