isn't HFT very math heavy and the actual solutions are done by essentially galaxy brain physicists/mathematicians?

seems like a very different skill than big tech coders.

In my limited experience of HFT, the galaxy brain mathematicians write terrible code. And there isn't much time for heavy anything at HFT speeds. So there's a lot of clever software and hardware engineering that is implementing what are often quite simple strategies. Those simple strategies are backed up by lots of clever analysis of course.

Right, the analysis is the heavy part. and yes the math guys themselves don't write the code, the coders do that part.

even if the impl can be quite involved, it seems rather strange that someone could make money from very simple strategies? seems to contradict common sense.

You can make money on average with simple strategies because you react faster to signals than the competition. Your order goes in first. You just have to be very fast.

I worked at a hedge fund that did a bit of HFT. They mostly took pride though in the fact that they made money using intelligence rather than just being really fast.

HFT is all about speed, and at the minimal amounts of time to react all you can do is simple strategies.