Why are you asking here when it is the platform providers (Microsoft, Sony, Value and Epic) that manages and controls how digital licenses function within their respective stores.

Tell everyone that you know that you know nothing about video game licensing without telling everyone that you know nothing about video games licensing.

As stated at the top of the page in the cross-save setup, purchases do not transfer between platforms. You must purchase them for all the platforms you intend to play on. This is normal behaviour and applies to all video games, not just Destiny 2. If you were to buy CoD for Xbox, would you expect it to work on PlayStation? No, you wouldn't. The same logic applies here. Purchasing content from one retailer does not give you access to it from different retailers. Full information at http://bungie.net/cross-save