Dexters warning is for those new folks that don't know any better and place the jack under the axle tube towards the center of the axle and bends it.
Place the jack under the axle tube next to the U bolt or the U bolt itself and you won't bend the tube and no it won't crush the tube either, on a std leaf spring axle.
I have a 12 ton shortie hyd bottle I have used for years on my equipment trailers axles from 1750 lbs up to 10k lbs.
Now if you have Dexters torsion axles then its a different story with those thin wall axle tubes.