It's a synthesis from diverse sources.
First, bible commentators don't deny that different styles are intimated by the terms psalms, hymns or spiritual songs, but, because musicology is not their forte, they can't say what each was.
It is not easy to determine precisely what is the difference in the meaning of the words here used, or to designate the kind of compositions which were used in the early churches. Barnes NT Notes
A psalm was a hymn, and a hymn a song. Still there was a distinction between them as there is still. Charles Hodge
Second, the nature of ancient Greek music. Hymns and dithyrambs were both sung in praise to the Gods, but dithyrambs were wildly exurberant odes,
bacchanalia. Descriptions which are not used for the greek form of hymn. (The word hymn in our language is much more generic than then.)
Third, the writings of ancients marking a distinction in the musical styles:
Now music was early divided among us into certain kinds and manners. One sort consisted of prayers to the Gods, which were called hymns; and there was another and opposite sort called lamentations, and another termed paeans, and another, celebrating the birth of Dionysus, called, I believe, "dithyrambs." And they used the actual word "laws," or nomoi, for another kind of song; and to this they added the term "citharoedic." All these and others were duly distinguished, nor were the performers allowed to confuse one style of music with another. The Laws, Plato
Fourth, but actually heading the list, the Scriptures themselves. The manner of interaction through psalms, hymns, and spiritual songs is starkly contrasted with the dithyrambic excesses of drunken banqueters in the Eph. 5 passage.
When an ancient Greek heard the word hymn, he didn't think of noble words in the style of a dithyramb. He thought of something in stark contrast to it.
