Exactly, Jubs! But while trying to maintain osmotic balance, the bloating is caused by an imbalance. By adding regular salt, this will pull more fluid from a lower to higher concentration. That's why epsom salts work here. MgSO4 actually helps return the fluids to the vessels rather than pulling them into the tissues causing even more of an osmotic imbalance. This is called "third spacing" of fluids.
I see what you are saying and I understand the physiological term of third spacing but I am not sure I understand it when it comes to a fish I guess, I don't know. I am just going on what my reference material treatment method is offering. I see that the common treatments according to mostly betta sites, after a quick google search, is MgSO4 as well...
Honestly the only way I can tell you for sure what works is if I was in a situation to compare them myself, you know me well enough to know I won't say one way or another if I haven't experienced it myself so with that said... I am basing that on my material I used for reference, if it is correct or not I can't honestly tell you as I haven't had the "opportunity" to try either way. I have a better idea instead feed your fish a good diet and keep the water quality up and you won't have to worry about diseases in your tanks!

I can tell you from actual experience this does work! I would be very interested in running an experiment to see which works better though, I don't why he would recommend the use of the wrong thing but to quote the author it says "pure salt or sea salt" so ...
I am sorry if my info is incorrect I am just basing it on my reference as I stated before. I would be very interested to see or run a test ran side by side or however, to see what happens. The hardest thing about a test like this is the fact that you would have to know what exactly the cause of the dropsy was so you could treat the fish for the cause of the dropsy and not play the guessing game most people do that know that dropsy is not the cause of the problem but rather an underlying condition that is causing malfunctions in the osmosis within the fish. So that leaves a lot of variables to successfully do a test like this as well.
I am not arguing what method is right or wrong as I don't know since I haven't had problems with disease in years and I can't run my own test. Just not sure why a disease expert would over look such a thing but it happens so till I have the experience to run a test ,I am with you just as much as I am with the author of my reference material.