Which is definitely a good thing.
The problem there is that he would need a somewhat smaller cylinder and that might not be readily available. (a waveguide diameter of 6.96 inches (177 mm) would give him a lower cutoff of 992 MHz.
So if he can find suitable metallic tubing with a diameter of 7 inches, he should be good to go)
But, if he builds it multiple waveguide-lengths long, as he indicated he might do, that will make the antenna
beamwidth narrower which will eliminate any signals his antenna isn’t aimed at. If the signals are not in the antenna pattern main lobe, but are very strong, they will be attenuated considerably.
If the interfering signal is in-line with the desired signal, then an inline filter might be required. The rolloff
of the antenna will help knock down the interference, but the skirts of the antenna response curve aren’t as steep as those of a cavity filter, hence the possible need for a filter at the antenna feedpoint.
The downside to a longer waveguide and thus narrower beamwidth, is aiming the antenna. The narrower
the beamwidth is, the more precise one must be when aiming the antenna at the signal source.