I think it would be better worded as this: if you create a port/version/mod of Shen, it must be able to execute spec-compliant Shen code.
Who decides whether a language I come up with is a port/version/mod of Shen? How much of Shen's features can I include without crossing that threshold? Does that threshold move if I can point to other non-Shen languages as the source of those ideas?
The license is for the software--the Shen implementation. If you don't use the Shen implementation provided, then you're not subject to the license. If you create a port/version/mod of Shen then you are using the software. The way ports of Shen are created is by implementing Klambda. The provided Shen implementation compiles Shen to Klambda, which your port then compiles to a host language. So, unless you really want to clean-room reimplement the entire Shen language (which is NOT how porting is intended to be done) then you're using the software that is subject to the license.
Who decides whether a language I come up with is a port/version/mod of Shen? How much of Shen's features can I include without crossing that threshold? Does that threshold move if I can point to other non-Shen languages as the source of those ideas?