Oh, I think it's ok if the manager, with an engineering background, spends a lot of time managing and not a lot of time writing code. They may be further away from understanding the details of the product, but that's ok.

I experienced this personally when I architected a product, the team grew and the product grew and before long there were modules that I had never worked on.

I don't think the manager of engineers needs to be as strong as the other engineers on the team, but they have to understand engineering on a fundamental level, and be able to look at code code and comprehend it, even if they don't know the whole product, or all the details.