In theory, sure, but the reality is that "development activity" revolves around "codebase". Plenty of projects have separate bug tracker, source code control and CI system. Most people likely prefer GitHub's all-in-one solution, especially for small-medium open source projects.
And do we not see how terrible this is as a result? Hitching your tent to corporations, especially American corporations, is always an exercise in misery.
They've already separated the wiki's version control, why not CI? And the issues/discussions are just vendor lock-in wrapped in a shiny UI.