I guess the point is if someone discovers a bug and opens a PR to fix it, then that person is, in a way, also a maintainer. They are “paying” for the maintanence of the project in time and effort.

No. A maintainer is someone who maintains the project. Fixing a bug is a great contribution and makes you a contributor to the project. But you need stick around the project for a while, fixing issues that keep the project running and doing tasks that aren't necessarily required for your use of the project to become a maintainer.

I would only say that they have become maintainers if they consistently do so, over time. Including helping out in areas which is not directly useful to them. And thinking about the whole picture, not just individual features and bugs. And also putting in the time when it is needed, even though other obligations are pulling at them. Of course it is somewhat of a continiuum. And it always starts with being a contributor. So that is on the right path.