IE is more than a web browser, it is a set of (coughs) technologies that run under the hood and that other applications use, borrow, steal, or whatever the correct terminology is. The browser part is just one facet of it that we can see, and I believe that is the portion that is being retired. The game doesn't actually call the browser up, but it does utilize some of its executables and the like. That is why clearing IE cookies and cache is always the Go To recommendation for when Launcher and in-game logins stop working.
To give it an analogy, although this may resonate better with Mac users than Windows, think of QuickTime. Even if you never used QT's own picture and movie viewers, the technology was always there for other applications that may have required it to use. Am not saying the game uses QT, again this is just an analogy and maybe not a very good one but the first one that came to mind.
