When you install RMT like suggested through composer, which makes a lot of sense, composer can put symlinks at vendor/bin to command line scripts within the dependencies.  In this case it makes sense if it links vendor/liip/rmt, since that's from what I understand the script that sets up the environment in which command.php is run. 
If RMT has __DIR__ there,  RMT_ROOT_DIR will point to vendor/liip/rmt, which is no good,  since that already contains an RMT script as well as RMTs own rmt configuration file. But when you run vendor/bin/RMT,  for example to initialise a configuration, you want it to use the environment of your project directory. Which usually and especially when you run init should be equal to your current working directory. 
Sorry if the PR was not clear enough about that, I thought the information in it and the issue referenced would be sufficient. 
I hope this reply is able to answer your questions, otherwise I can paste you a few examples once I'm back in the office tomorrow.