You aren't wrong. They are threaded. That's just Jay being Jay.
sure its threaded, but some will pry the cap off,! so I mentioned it,
since some folks tend to do the backwoods hillbilly approach to patching things...
so do it the right way and fill it thru the fill hole on the side of the transfer case!
get a gear oil pump -problem solved.
another quick easy way is all the gear oil plastic quart bottles have a fill snout on it,
plenty in yer garage if you look around . . slice the spout off half way down the cone, stuff a short piece of 3/8 hose on the end of the fill spout, [I have used clear plastic hose and fuel hose] stuff the other end of the hose in the tc fill hole and squeeze the bottle.
keep 4 quarts of gear oil and the hose in yer box of essentials in the back of yer scout, so if you're ever out on the road somewhere and develop a leak you can easily refill the tc/ trans or diff.