The Stop Killing Games’ end goal is that governments will implement legislation to ensure the following:
Games sold must be left in a functional state
Games sold must require no further connection to the publisher or affiliated parties to function
The above also applies to games that have sold microtransactions to customers
The above cannot be superseded by end user license agreements

It's asking for games to remain functional, not for source to become available. Even the Video Games Europe reply linked in the article mentions that the main problem, for them, is "keeping servers online indefinitely":

It appears to be a combination of a requirement to provide online services for as long as a consumer wants them^[It's not that, you fucking douchebags], regardless of price paid, and/or a requirement to provide a very specific form of end-of-life plan where the game is altered to enable private servers to operate^[This is the easy solution and perfectly doable, no matter what bullshit they try to use as an excuse. Don't want to keep your own servers anymore? Release the files needed to run private servers, update files so any game can connect to any private server, done.]. We do not believe these are proportionate demands.