Historically WW2 airbursts came in 3 flavours.
Proximity or VT (Variable Time) fused shells which had a miniaturised radar unit that triggered the charge a set distance from the ground. Only available in combat zones from the end of '44 for the Allies. Originally designed for AA shells (down to 40mm Bofors), put in plain artillery shells as well.
True shrapnel shells which were designed to burst in the air forming a forward and downward cone of balls, a bit like a canister blast. Usually used a variable length powder train to trigger the bursting charge. Early WW1 field guns usually had exclusively shrapnel shells, later supplemented or replaced by HE, but shrapnel was still in use in WW2.
Standard HE triggered by trees or light overhead cover. A fair few "Bulge" accounts that I have read mention this as a very real problem, for example. How solid the triggering item (branch, roof slate, etc) would have to be, would vary between shell designs and maybe even production batches. A hit on a solid trunk or substantial branch should trigger all, lighter branches ... depends.
The last is what ASL's airburst rules simulate. While the <= 50mm mortar rounds might not be the most effective examples, there would be some effect and those only have 2 FP in ASL anyway, so I don't have a problem applying the rule.
ASL was written in the early '80s when a lot of WW2 history was under researched or original materials still hidden (eg Soviet archives, ULTRA), but this was, in my opinion, a reasonable approximation to a real and significant combat phenomenon.