I believe that you are a bit confused about how things work in Android.
There is no “lemmur’s internal browser”. At least not as far as I understand. What you are referring might be Android’s built-in browser. It opens links in any app, not just in Lemmur. Opens as in it displays a website, but when you look at your open apps, the browser is not a separate app, but instead appears to be the original app from which the link was opened.

In my case, I use Firefox Nightly, and I have it set as phone’s default browser.
When I either click on a link in Lemmy, or when I use the “open in browser” option, Firefox opens up as a separate application and displays the website.

So I’m not sure what exactly you need to do in order to set up the desired functionality, but I am telling you that it can be done.

also the “open in browser” is not what you want, since you want to open “links in posts”, but the open in browser thing opens the web interface for Lemmy, not the link that is in the post.