I might have been unclear. To emphasize, I said “a property”, and you’re talking about “the property”. Once someone understand that the property is not lt(this_element, next_element), but instead !lt(next_element, this_element), I don’t see what confusion remains.
And I’ll argue that you don’t need to look at the implementation to know that that’s what’s happening, if you understand that the default value of lt is isless, and that issorted accepts any function for lt.