Gliders are dumb, use balloons
If you heat up the air, that is going to dissipate quickly due to convection and winds and whatever. The first people trying to get airborne with hot air didn't use gliders. They used balloons. As an added bonus, if you are using primitive technologies, people went up in balloons in the 1700s, so the technology level is about as low as you are going to get.
Heat loss in a hot air baloon
Conveniently, Adriana Llado-Garcin, who I never met but now I love, did her Master's thesis on a Heat Transfer Model for Hot Air Balloons.
I started doing some fancy math, but then realized a few things. First: 1000 W is nothing. The burner studied in the paper was 3.2 MW.
Second, 1000 W is really nothing. A hot air balloon presents a surface area of about 400 m$^2$ to the sun, which on a sunny day will warm it with 1000 W/m$^2$. A dark colored balloon will absorb 90% of this radiation. Therefore, heat input to a large-ish hot air balloon from the sun on a sunny day is about 360 kW. So putting your balloon on a field on a sunny day is about the same heating as 360 of your pyromancers.
I didn't end up doing any math out, but the discrepancies in power numbers impressed on me that this scheme will not work. If you have a better chance of firing a hot air balloon by leaving it in the sun, and you certainly won't lift off by leaving it in the sun, then your pyromancers can't reasonably launch a balloon.
And if they can't reasonably launch a balloon, what chance do they have of keeping their glider airborne? I'm calling this myth busted. Better tell those pyromancers to step away from the cliff.