For example, I can change my username here on this instance. So long as my email id remains the same, my account is operable.
But GitHub doesn’t allow me to change my username. On top of that, GitHub forces an identifier on you when you create your username.
“Open source” means that the code they used to build software is freely and publicly available. (And more broadly, if any given IP is released to the public.)
It says nothing about how user accounts are managed on a given platform, even if that platform is open source.