If we have DRM with some private key, then I guess your idea is I download the game files and some private key and that allows me to run the game.
If I can send you the private key and the game and it allows you to run the game with no further inputs, then the DRM is trivially broken (even without open source).
If it does some online check, then if the source is open we can easily make a version that bypasses the online check.
If there is some check on the local PC (e.g. the key only works if some hardware ID is set correctly), we can easily find out what it checks, capture that information, package it, and make a new version of the launcher that uses this packaged data instead of the real machine data.
If you use a private key to go online and retrieve more data, having it be open source makes it trivial to capture that data, package it, and write a new version of the launcher that uses that packaged data.
Basically, DRM requires that there is something that is not easy to copy, and it being open source makes it a lot easier to copy.
