Note that I am justifying the definition of issorted by discussing the function isless. Julia (Base and software ecosystem) is unlike any other in my experience in how well it does generic programming. By design, isless and isequal is more “fundamental” than the <= that you are advocating for.
We are lucky when outcomes of careful design for generic programming aligns with the expectations of every user, and we appear not to be lucky this time. It is obvious to me that the solution is better documentation, instead of re-working how comparisons are done in Julia.