On similar forks I've dealt with, the trick they're playing is that the coil spring inside is the part that joins the upper to the lower.
Hidden inside the lower, there's a threaded boss welded in, and a bolt threaded into it. It's an M5 cap screw from what I've seen, although there could be variations. You access it by poking a 4mm allen socket on a long extension down through the coil spring. This secures an unusual looking round "block" which has unusual screw threads form on the outside. The fork spring threads over it, and the whole assembly is secured to the lower with the cap screw.
There's another such "block" that secures the top of the spring to the upper. If memory serves, on the one I've taken apart, you get that threaded out of the spring with a 5mm or 6mm allen fitting that's exposed once you remove the preload adjuster.
The preload adjuster comes off with a plastic wrench that RST sells. If you're willing to accept some marring, you can typically get them off and reinstalled with improvised tools.
I have no idea whether replacement wiper seals are available, and doubt this is worth dealing with even if they are. It's grease-lubricated inside and the grease is thick and lasts a long time, intended never to be opened or serviced.