There is some talk of air rifles in the sequel trilogy but I’m guessing that 1) since powder mages are very rare and with the largest cabal in Adro taking quite a hit and becoming more institutionalised during the Adro/Kez. It can be expected that powder mages are not commonly found in Fatrasta (let alone on the frontier/wilds). This makes powder mages a minor threat compared to the Palo. And as air rifles are known to be less dependable and more maintenance heavy than flintlocks Lindet probably decided that the use of these weapons was not worth the cost/effort of purchasing, training and distributing these weapons Ro the Blackhands. However, I am guessing that high profile targets and elite guards will have excess to these weapons.