When using the Xbox interface to access your account, you will use FFXIV coins.

When using other methods, such as a web browser on your PC, other payment methods will be available.

It's that simple!

It does seem to suggest that if you don't have other platforms attached on the account, then it might still want you to use the Xbox + FFXIV coins for payment, but since you said you have other platforms, you should be able to handle it all via the website.

You should be able to pay the subscription fee without these coins at least. Since buying expansions is platform-specific, you may end up buying the Xbox ones via the Xbox interface, which would still require FFXIV coins, but you could also try buying it from a retailer such as Amazon or the Xbox website for a normal currency.