Sam, perhaps Melach Sedomis, although rare, was found in that region. Tosafos, being in Europe, didn’t get from that salt at all.
The Aruch Hashulchan might be implying that the particular Bal Tosfos who said that was on a mountain. It is true though that the Poskim who are Sod oriented to say to continue with this Halacha. The Maharal also says this. He says that Melach Sedomis is itself a reference to more than just the sharp salt.
We have a rule that Ein Beis Din Yachol Levatel … Ella Im Gadol Bechachma Ubeminyan. Tosafos would not be Mevatel a Derabanan. The Gemara in Eiruvin said that Shlomo Hamelech had 1005 reasons for all Divrei sofrim. However, what we do find is that Tosafos would refer to a Derabanan that was not practiced, and say that due to the new circumstance we do not need to reinstate it. If even a small part of the original reason still existed, Tosafos insisted that it must be adhered to. This we find by the case of burial on Yom Tov.
The Tosafos on clapping, jewelry and Mayim Acharonim, among others, are addressing the fact that it wasn’t practiced.
The concept of Halachos of the Gemara not being followed to the T is not new. Tosafos says that we sometimes follow Sefarim Chitzonim (such as Maseches Smachos, Sofrim). There were obviously different Mesoros which might have originated from Eretz Yisroel or whatever, that we got from. There is a framework for how to deal with such things in retrospect, which we can learn from Tosafos.
Talmidei Chachamim, who study and know Kol Hatorah, and care about it too, are the right choice for giving an insight into these matters. Outsider’s perspective might be fun to read but cannot be used seriously.
The Chazon Ish knew all these Tosfos, and yet remained Frum!