> my reading of a lot is always more than 50%, which implies to me that more than half of the engineers at half of the companies have average IQ or lower - which seems unlikely.

That depends. It's unlikely that they have at most the average IQ of the general population.

But if we assume that engineers as a group are smarter than the general population, it follows immediately that over half of them are below average among engineers. If you mostly work with engineers, that will probably look like "below average".