In a medieval world with current knowledge of aerodynamics, yes. Nature provides enough tough materials to build a structure that a single human could use to glide a bit.
1) With current knowledge, a design could, within a few months and with small loss of life and limb, be brought to perfection, but with then contemporary knowledge, it would be very hard, and could only be bought by many years of experimenting, and a host of lost pilots. You'd need to get medieval to find more testpilots. Hey...
2) This is about a glider. Not a good one, either. Optimistically a glide ratio of 3:1? (Every 3 meters you go forward are bought with one meter height loss (https://www.britannica.com/sports/hang-gliding)). High ground was coveted even then, So having to find an accessible yet steep hill overlooking your target, undefended, and then schlepping your gear up there, to surprise the enemy below? Not that many chances for that.
3) Using mechanisms to gain height, like winches, counterweights or mooring in a stiff breeze would add many more years of testing, invalidating many of the pure glider designs because they now also be able to survive these forces.
4) Birds, stones, arrows, and many other flying things were known in medieval times, so a commander getting credible info about the enemy being able to glide 'like a chicken with clipped wings' (or some such) could identify possible ambush-places that were made viable by such technology. And you have just been chucking people from your castle walls tied to tents for years now. This will not be a secret. People will be somewhat prepared.
5) Whether by time travelling aeronautics engineer (cum carpentry enthusiast (with a pinch of not getting burned as a witch)), or pure aristocratic bull-headedness, you now have gliders that can be packed like tents, a mobile launch system, fighter-pilots, and a bird-related nickname. Whence now?