The thing is interns and part-timers are typically not counted as part of the official headcount. Relic for instance has hundreds of testers - who are all part-timers - but has an official headcount of 120, which is only slightly more than double Eugen.
$40,000/year moreover is the actual national average for France. If they offered their regular employee programmers much less than this, then they wouldn't have anyone working for them especially since they are based in Paris. Indeed, I suspect any labor shenanigans (including using interns for an extended time period almost like regulars) in Paris would very likely result in a quick labor suit.
Finally, and very bluntly, the figures I gave were already pretty low. It would not be unusual for a game the size and scope of Steel Division to cost at least $10M nowadays, thus requiring 500,000 copies sold to break-even on Steam. But hey, Wargame 4 is surely around the corner right? Eugen must surely have stacks of money somewhere because they use interns just like every other game company does.