I was horrified and I said to them 90% of the code that is running on board your aircraft is probably written in assembelr simply because of the speed requirement ... <p>

... if that is the case. If they have adopted some subset of the MISRA rules, they are going to avoid assembly as much as possible. Same thing goes if they want to keep their code portable, or if there are regulations on safety-critical systems. Usually, the requirement isn't that they have to be fast, but that they have to work.

Or maybe they don't have anyone who is familiar with this specific chip ?