The word matsya arrives in this corpus already carrying weight. Matsya is the first avatar of Vishnu, the fish who warns Manu of the coming flood and tows his boat to safety, a story told in the Mahabharata and in the Puranas and among the best known in Hindu narrative. Matsyendranath, whose name means lord of fishes, is the figure the Nath tradition treats as the first human teacher of hatha yoga, and he already has a posture of his own in the corpus.
Neither surviving manual draws a line between the posture and either figure. The Gheranda Samhita names the shape and describes it, and that is all. The atlas therefore records the resonance without asserting a derivation, because a shared word is not evidence of an intended reference and the difference between the two matters in a reference work.
The temptation to join them is worth naming, because it is the ordinary way folk etymology works in a devotional tradition: a name is heard, a familiar story is nearby, and the two are connected by a later teacher explaining a posture to a student. That process is real and it has shaped how many postures are taught. What it does not do is tell us what a seventeenth century compiler meant. Where the sources are silent the atlas says so, which leaves the question open for anyone who finds evidence the compilers left elsewhere.