A bit off topic, but commenting on footnotes is fair play: I don't believe that 'let' is in fact autoantonymic.

A let is more-or-less lost, as a noun in English, but in the form inlet and outlet it remains ubiquitous. In "let or hindrance" we might observe that a right of travel (a let) might well be a hindrance to other uses of the property.

A let in tennis is so-called because you 'let' the server try again. The sense is clearly one of permission to the player, not hindrance by the net.