I have no problem with the claim. I've hit an 18" plate @ 300 with the ex's Glock 19, one handed. Repeatedly. Not every shot, but enough to know it can be done. Two handed, the number of hits can go up. A difference in bullet type makes a noticable difference in the amount of sight needed to make hits. WWB 115's shot flatter than Rem HP's. Two handed helps a lot, as does a good rest, or kneeling. I've made 6 for 6 @ 300 with a 22 Smith on an 18"w x 36"h plate, kneeling two handed, and 5 for 6 with a 4" 29. It isnt anything spectacular. I think I could teach any decent pistol shot to do it in an hour. I havent shot farther, 300 was fun, and interesting. I know a guy that shoots farther with a sixgun, up to about 800 on occasion, just seeing what he can do. I believe he said he could hit a half sheet of plywood @ about 600 fairly regularly with a heavy loaded 45 Colt.
Couple of points worth mentioning, a .40 @ 500, whatever the comparable paper energy, has more momentum than a 22, and will punch through more steel or whatever than the 22. The 22 also has a soft bullet, and can deform easier, so may not penetrate your particular drum of choice reliably, tho it seems to be able to poke holes in your neighbors drums.
I have no idea what the paper trajectory is on any pistol load, and it really doesnt matter, you don't shoot pistols at long range on paper trajectories, or by adjusting sights for it. You do it by holding the front sight up above the rear some amount. Trial and error shows you how much. The more you do it, the more you can do it well. The numbers mean nothing. For the ex's glock 19 with wwb 115's, I held about 2/3 or so of the white dot above top of the rear sight, with the 300 yard plate on top of the front sight. That made hits. The HP loads took a little more front sight. I have no idea how much "holdover" or "drop" it had, but I can hit the plate with that sight picture. Two handed takes more front sight, as the gun is held down tighter. I've shot other glocks at the 300 yard plate, and after a few rounds, could range it in and hit. Smith sixguns have much better trigger actions, and are generally easier to make longer hits with.
"Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat." -Theodore Roosevelt-