I agree port knocking is a direct violation of Kerckhoff's principle. However, the proposed solution has non-discoverability from unauthorized sources which isn't necessarily in the threat model of OpenSSH or general cryptography. I do feel like this is potentially a desirable trait. I elaborated a bit more here[0], but I'm curious if you have any grander thoughts on how this could be approached
[0]: https://news.ycombinator.com/item?id=49307986