Preql: a new relational programming language that compiles to SQL

> Preql uses an “Interface-Protection Clause” on top of the MIT license.

Can you license an interface?

Does that mean that anyone interested in possibly implementing something like this shouldn't look at this project to avoid the copyright risk? That seems antithetical to open source - why not just keep this fully propriety and private if you do not intend to share the ideas behind it?

This seems like a cool project but, with a license like that, I'm not even going beyond the README.