As a kid, Boimler was either orphaned or his family “volunteered” him to a group of terran rebel scientists who were making augmented super soldiers to fight the Klingon-Cardassian Alliance. I haven’t decided if they’re trying to make soldiers for a new stronger empire or just desperate for any advantage over the Alliance, but I have decided that this was group that augmented Bashir. Being part of an earlier batch, Bashir may not have gotten as extreme augmentations.

Boimler gets the same augmentations that he got in the prime universe in terms of intelligence and whatever let him finish all his tasks during the whole buffer time thing and survive explosions (also the unintentional ones that make him skinny and neon purple). In the mirror universe he also gets augmentations that make him significantly more athletic, but he still looks really frail.

Only the personality augmentations don’t take and he doesn’t turn into a cold, ruthless warrior like the rest of the augments. Which makes his anxiety worse as he feels like he’s letting everyone else down. Finally as a teenager he decides he doesn’t want his failed augmentation to drag everyone else down and he runs away, eventually winding up on the Cerritos.