Where on Earth does this sentiment come from? I've had people make AI contributions to my open-source projects and while they may be perhaps less technical on average they're every bit as well intentioned - possibly moreso since they're interested in improving the project rather than establishing a little fiefdom.
using AI is not vibe coding. the difference is whether a developer takes responsibility and the according measures before releasing/contributing or not.
The "developer" could be a bot, or a tool-assisted criminal or activist, and they might be using LLMs to enhance their ability to add stealthy malware, plant patent or copyright violations, impose not quite shared technical choices, and so on.
Many open source projects ban AI contributions just because they are an efficiently low effort way to waste the time of the actual maintainers.
E.g. I've had people find genuine bugs and then basically paste their entire Claude conversation into the issue. Very annoying.