I think you’ve confused that “incompatibility”.
Here’s the basic gist of it:
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If you buy from MS, you can run the sim on both Windows and Xbox. With a few exceptions, all 3rd party marketplace content you buy will be available on Windows, and most on Xbox. Some marketplace content is only PC compatible for various reasons. Marketplace purchases are not transferable between the MS store and Steam however.
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If you buy on Steam, all marketplace add-ons can be purchased and run fine. However, if you also have an Xbox, you WILL NOT be able to use the sim nor any of your marketplace add-ons without re-buying everything on Xbox.
My personal recommendation. Take that as you will:
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If you’re already a Steam user, get it on Steam. It’s simpler, keeps all your games in 1 place. You don’t get your main MSFS folder DRM locked, and you don’t need the plethora of middleware apps MS forces you to use when using their store. You won’t have it available in the MS Store or Xbox, but you can buy any content in the in-sim Marketplace just fine. The transactions and payment will take place via your Steam account. There’s no incompatibility.
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If you have an Xbox or plan on getting one, the MS Store is the best choice. Although beware that it’s a real Rube Goldberg type of setup where you need the MS Store, Xbox Live, and all MS’ midddleware apps kept up to date, working, and logged in for any of it to work. For most people, this isn’t an issue, but some people have strange problems.