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

Iris Cohen schreef
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.

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Leaving aside the issue of whether fungi are plants, mycorrhiza have complex
relationships with plants and certainly influence how well the plants they
interact with are doing
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Also I don't see any connection between mycorrhiza and allelopathy.

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I imagine "allelopathy" is one of those words having multiple meanings and I
would rather not get in a discussion about it. I would not preclude any of a
number of possible connections.
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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.

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

Where on earth did you get that idea?
Iris,

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Actually I don't know much about orchids, but I am very dutiful. Since this
is a favorite story of one of the world's leading orchid taxonomists I am
accepting it without question.
PvR