Siddha means accomplished, perfected, one who has arrived. In the medieval religious world the hatha texts come from, it is also a title: the siddhas were practitioners understood to have attained a perfected body and the powers that came with it, and they form the milieu in which alchemical, tantric and yogic technique were pursued together. David Gordon White's The Alchemical Body reconstructs that world in detail, and Geoffrey Samuel's Origins of Yoga and Tantra sets it inside the longer development of Indic religion.
Naming a posture after them therefore claims something. It is not the posture of a person who is comfortable sitting; it is the posture of the accomplished, the seat of those who got where the practice goes. The Pradipika reinforces the claim by placing it above the other three principal postures and by attaching to it the promise of liberation.
The same root produces siddhi, the powers the tradition holds a practitioner may acquire along the way, and the ambivalence about those powers runs through the literature: they are evidence of progress and a distraction from it. That tension is part of what the name carries. The atlas records the claim the text makes and the world the word comes from, and leaves the claim standing as the tradition's rather than restating it as a finding.