You have to log in to GitHub in order to create issues on GitHub. But that doesn’t mean you must have a GitHub account to report issues to a project hosted on it. Emailing the author with an issue report (and a patch) works as well.
Then you find that GitHub forbids multiple identity contexts for free accounts in their terms of service ("One person or legal entity may maintain no more than one free account"), so if multiple people want you to do things on GitHub but you don't want to mix the activities together, your options are to pay for a monthly subscription (possibly forever) or break the rules and hope your ability to continue participating doesn't suddenly get taken away if they catch you.
Centralization of interaction by type creates social paradoxes like this.