I can't seem to replicate it. But you can go through there as demo with all the weapons, not just the sticky launcher.
Ok it's weird, while I am typing this I am trying it some more, but now it suddenly only works with the launcher, it seems.
Ok, more testing, I think I know how it works: Initially you can only go through there as demo with the sticky launcher, but once you went through there once, after that you can change weapon or class, as long as you don't die/kill yourself. The teleport "remembers" that you went through there and still allows you to go through even though you're maybe a soldier or a demoman with a wrong loadout.
I think that might be kinda bad for servers as well.
You can check the map I posted, I guess you can still pass that with bhop though.