Perhaps the engineer was contributing something crucial to the product design, you just couldn't grok it.
Product design is so much more than UI/UX, fwiw.
I worked on one product which involved a massive store of XML documents and a suite of somewhat complicated serializations and transformations. It was the right architecture for the project, for sure. And it was definitely the fundamental gene of the product; all of the product's design had to flow from this XML-heavy information architecture and backend.
But most of the products I've seen are not so unique. For the vast majority of web products out there, simple CRUD apps backed by a relational database, I don't think the information architecture needs to be front and center in product design conversations. People can make assumptions about this kind of app and they usually won't be too far off the mark.
Just last week, I had the very first design meeting for a new project and a veteran engineer was invited. He didn't even let the presenter finish before talking about how the backend should be built and how much time it would take. That wasn't helpful.
I think engineers who are good at design are immensely valuable, but most engineers suck at it.