The allen wrench is only to hold the post. The ratcheting socket breaks the nut loose.
Tool stores have much more robust allen wrenches than those at big box stores. Mine have a side that’s 6" long for better leverage, and I’d bet that they’re made of a better steel. If you have space, you can also use an allen tip in a ratchet wrench to hold the post while turning the nut.
The nut will be a b**** to turn while holding the post, and it won’t loosen up until it’s almost off, but it CAN be done.