Projectile weapons other than bows do create projectiles, and seem to be affected by projectile damage perks, but not projectile speed perks. This includes things like Guns Without Roses firearms, Immersive Engineering revolvers and railguns, etc.

Because the only way to track it normally would be off the onArrowFire event, which then modifies the speed.

Those mods must be using their own custom code for their projectiles, and not passing the arrow fire trigger.