so, from my understanding of the photo people are able to change the ingame text
how does that allow people to hack steam accounts?
So with source code, you have total freedom. You can change all sorts of things like add ability to freeze a player in place or render maps in wireframe or change reload speed, add some telemetry-collecting traces on other players etc. You compile the project and you get your very own, modded .exe
In theory, you can still play on the same servers but with your juiced-up executable.
In practice, you maybe can do some juvenile shit like put overlays on people's screens and do some griefing but to hack into someone's account you would probably need Steam's source code to do that.