Shorter barrels of a given diameter are a lot stiffer than longer barrels. They flex less from the admittedly limited shock produced in air rifles, compared to PBs; and so short barrels suffer less from "harmonics".
Now, it might help to define the length of a "short" barrel, as well as a "long" barrel. Too short and muzzle blast can buffet an airgun projectile, cancelling any barrel stiffness advantage - unless you use a good air stripper. Also, as mentioned above, barrel length and tune need to be compatible, else all bets are off.
If you want a long barrel that does not require a lot of attention to harmonics, to enable shooting at high power, then make that barrel stiff. This can be done by using a large OD, or by tensioning a thin barrel in a steel, aluminum, or carbon fiber tube. Using different materials requires measures to tolerate different thermal expansion, so that barrel tension is maintained over a reasonable ambient temperature range. Else, harmonics drift with temperature, along with POI and potentially, grouping ability.