Agreed, there is very little challenge in NOT buying artillery or airplane DURING a scenario, while playing a campaign game. Maybe when playing a single scenario, say Bagration for example, you have strong reasons to buy artillery or airplane during the scenario to make up for the high casualty rate, but not for campaigns.^ seems that's the solution. We can argue if this is then a "bug" or "working as intendend", but then it is a rather odd achievement with a very misleading description.
It seems everyone read it rather as "never buy artillery or fighters" and not as "never buy artillery or fighters AFTER deployment".
I would sugggest to either change the trigger to include not buiyng thos units even duing setup. I'm not sure if the individual year of GC count as one campaign each for the achievements. But you can easily buy additional artillerys and airplanes during GC 39 for example, to get them ready for GC40.
Or at least clarify the achievements wording, as it is so unusual, hence prone to misunderstanding.