Lot's of people use apps that don't expose a link, share buttons are great and even better when they use standards like your OS's share functionality.
There are so many products on amazon sharing a link to one specific product and having someone else open it shortly after sounds like a high enough confidence.
Again just a hunch.
That said, I'm curious as to why someone with enough technical sophistication to be posting on HN browses Amazon with a native mobile app instead of a web browser.
Why do you, as someone with enough technical sophistication to be posting on HN, not understand the words? This is sarcasm, you don't need to answer.
Firefox for Android syncs with Firefox desktop so there's a shared history and I can easily send tabs from one to the other. It's also easy to copy proper URLs instead of shady shortlinks that often contain affiliate codes and trip spam filters. Finally, I don't want to give Amazon a foothold on my devices; the browser has a stronger sandbox than native apps do.