The problematical definition would be air weapons but not firearms. It would be a perverse interpretation given the definitions in FA 1968 Section 1(3)(b) , and the "air weapon / other firearm" in S19.
The only thing giving me slight pause - and given the particular loathing for gun-shaped toys North of the border - is the meaning of "air weapon" in Section 1(4)(a) of the Air Weapons Licensing (Scotland) Act 2015.
"an air weapon which is not a firearm (within the meaning of section 57(1) of the 1968 Act)"
That refers to the 1J lethal barrel limit. This legislation was written before the Policing and Crime Act 2017 and so couldn't have referenced the S57A exemption for airsoft guns.
But PCA 2017 didn't amend the Scottish legislation to say "57(1) or 57A", and neither has Holyrood now that 57A exists. Can Westminster legislation amend Scottish Acts? Don't know! But if I were working for the Procurator Fiscal (presumably after having traded my soul to Satan for a bag of Tangfastics), I'd bang the table and say "Ah hah! That clearly shows that Westminster-and-or-Holyrood meant to draw a distinction between <=1J airsoft guns, which are neither firearms not air weapons, and >1J airsoft guns which are not firearms but which are lethal barrelled air weapons for the purposes of the Scottish Act. Hang him, your honour! Hang him, then send him to Barlinnie!"
It would be a preposterous thing to claim, and I present it as such. There would be no reasonable prospect of conviction unless you misrepresented the law and/or manipulated the evidence, which is exactly what the Scottish Fiscal has form for doing with airsoft guns.