"designed to be small and efficient" – adding a layer on top of SQL is necessarily less efficient, adding a layer of indirection on underlying optimisations means it is likely (but not guaranteed) to also generate less efficient queries.
"make developing queries simple" – this seems to be just syntactic preference. The examples are certainly shorter, but in part that's the SQL style used.
I think it either needs more evidence that the syntax is actually better or cases it simplifies, the ways in which it can optimise that are hard in SQL, or it perhaps needs to be more honest about the intent of the project being to just be a different interface for those who prefer it that way.
It's an interesting exercise, and I'm glad it exists in that respect, and hope the author enjoyed making it and learnt something from it. That can be enough of a why!