It does not seem to be true. The Knots github lists someone called Retroplex as a contributor other than Luke. It also lists other usernames as code authors, but so far the other ones I've looked at apparently wrote it for Core and then Luke copied their code to Knots too. That's worth something, but in my book it does not count as an intentional Knots contributor unless they looked at Knots while writing it and intended to get it into that codebase.

Retroplex seems to fit that description, which means it is not "just" Luke. I am hoping to find at least 13 additional contributors in the sense which I describe in this post -- that is, people who write code while intentionally thinking about how it would work in Knots and working with Knots to be sure their code will work as intended there.