I imagine it's a similar case as to why Destiny still had Red Legion troops showing up in the overworld despite that faction looooong being "dealt with" in the story... They were simply said to be stray remnants.

Anyway it ain't something that Costa would care about very much nowadays. The average Garlean grunt looks like a dude wearing a bike helmet, a sweater, tights, and whatever class-appropriate LARPing gear they're decked out in.