Open source principles have to do with the source being available and users being able to access/use/modify the source. Chrome is an open source project.
To try to expand "open source principles" to suggest that if the guiding entity is a corporation and they have a heavy hand in how they steer their own project, they're not meeting those principles, is just incorrect.
The average open source project is run by a person or group with a set of goals/intentions for the project, and they make decisions about the project based on those goals. That includes sometimes taking input from users and sometimes ignoring it.
what's missing is social infrastructure to direct attention to this (and maybe it's missing because people are too dumb when it comes to adblockers, or they are not bothered that much, or ...)
and of course, also maintaining a fork that does the usual convenience features/services that Google couples to Chrome is hard and obviously this has antitrust implications, but nowadays not enough people care about this either
I don't know that road maps are any more or less "community driven" than anything else given the nature of their structures, but one can draw a distinction between them and the degree of corporate alignment like React (Facebook), Swift (Apple).
I'm agreeable enough to your characterization of open source projects. It's broad but, I think, charitably interpreted, true enough. But I think you can look at the range of projects and see ones that are multi stakeholder vs those with consolidated control and their degree of alignment with specific corporate missions.
When Google tries to, or is able to, muscle through Manifest v3, or FLoC or AMP, it's not trying to model benevolent actor standing on open source principles.