There is so much hyperbolic STUPID here it's unbelievable.
Leaving aside any evaluation of the (sometimes questionable) judgement of folks who consider ANY means whereby "somebody else" can get "more stuff" in the game easier than they can an "exploit", some of them even going so far as to make arguments readily paraphrased as "they play more than I do, I need to have as many shiny things as they do to make it fair!"....
The real question is "Is player X cheating - not "exploiting" - by doing Y"
We've all got opinions on that but the only one that matters is FDs. Doesn't matter if YOU think that using mission stacking to farm credits and rep quickly in Quince is an "exploit" - FD haven't declared it a "cheat" so it isn't. It may be lame, it may be poor sportsmanship, it may be a result of a game mechanic unintended by FD that they will subsequently nerf, but that doesn;t make using it "cheating" and therefore subject to sanction.
It's like combat logging - Exiting to avoid ship destruction via the menu and the timer may be lame but it isn't cheating, whereas causing an ungraceful exit to circumvent the timer IS cheating. If they catch you they'll do something about it.
There have been credit cheats in the past where it was possible to exploit a bug and gain credits, rather than just lamely but legitimately stacking missions... FD rolled back those transactions and the cheated credits went away. On the other side of a similar approach shortly after engineered weapons were introduced a bug cause many NPCs to have multicannon rails and player ships were exploding like popcorn... Those transactions couldn;t be completely rolled back but the players impacted got the full purchase price of the ships they'd lost back. FD do do something about bugs like this.
This engineer bug is more like those credit bugs than it is like the situation at Quince. At a minimum, those who used it can expect to lose the mods they got that way. Since in order to benefit from it, it takes specific action to DELIBERATELY exploit it, it's a much more egregious cheat, and should indeed attract heavy sanctions. There's absolutely no comparison to the lame but harmless mission stackers.