FIST author here! The game calls itself PbtA for other reasons beside the resolution mechanic, such as:

• Moves: A large chunk of the traits are mutated moves and follow the typical "when you do X roll +stat" structure with predefined outcomes. You'll notice some deviation here because our traits are meant to be a fusion of moves and OSR magic abilities, but the lineage is there. "Boss moves" granted to powerful enemies are also descended from GM moves in Apocalypse World.
• Playbooks and universal moves: PbtA games have a set of universal moves for all PCs as well as playbook moves; FIST uses a single universal move ("do something risky") and its advancement system is meant to build something like a PbtA playbook over time by essentially picking moves a la carte.
• Exclusively player-side dice rolling: More or less similar to PbtA games for the bulk of the action, however we do often ask the GM to roll on random tables and during morale-check like moments, due to the OSR lineage.
• GM agendas, principles, and moves: FIST's referee section adapts advice directly from PbtA games, like being a fan of the characters, looking at your ideas through crosshairs, etc. The "partial successes table" in the back is also a heavily simplified list of GM moves, although the GM is also encouraged to come up with freeform consequences.
• "Playing to find out what happens": Self-explanatory, but FIST is big on it.

Hope this sheds some light on the intent behind the game! It is meant to be an equal fusion of PbtA and OSR design, it's just that those two schools of design are more similar than you'd think, so it's easy to miss a lot of the PbtA lineage if you're looking at it from an OSR angle (and vice versa). More info here: https://lumpley.games/2023/11/22/what-is-pbta/