I don't know, I'm just reporting my experience as a user. I don't develop for Android or know anything about what Mozilla says about it.

Feel free to reproduce it yourself: block third party by default (for example), open some broken page, observe it as such; 'open in browser' from 3-dot menu; amend settings to allow whatever it needs; back out, observe it functional.

Open in the browser is not the same thing as an embedded web view. Take something like an RSS reader. You click on the link and it shows you the web page inside the app without taking you outside of the app.