This isn’t true. The game is priced the same on both stores unless there is a sale running. For example, the Microsoft store version has been 50% off for the last week while the Steam version has been full price. I expect that will change once the Steam sale starts today.
As far as ancient coins go, most of the bundles are slightly cheaper on the MS store for me (CAD) by a $1.
In Indian currency, since the launch of the game on steam, the price of the game on steam is less than half of the price of the game at MS store. Even now, when a sale is going on MS store, steam is selling SOT with much cheaper price even without having any sale for SOT.
I have already bought the game long ago from MS store, so, its pointless to discuss the prices of the game. But, the in-game micro currency (Ancient Coins) price at Pirate Emporium should be at a competitive price, as steam is selling Ancient coins bundles at much lower rates, so that we can also buy Ancient coins for the Pirate Emporium cosmetics or if any Battle Pass gets released for this game in future.