Without any prior professional experience, is an extensive open-source/[other non-professional software development related experience] portfolio perceived as more valuable than a degree to employers?
That entirely depends on the employer, but in my anecdotal experience that has been the case. Especially in more recent years versus the start of my career (nearly 20 years ago).
The reality is that Computer Science is useful for building strong engineers over the long-term, but it doesn’t at all prepare you for the reality of working in a team environment and contributing code to a living project. They don’t even teach you git as far as I’m aware.
Contributing to open source demonstrates a lot of the real-world skills that are required in a workplace, beyond just having the comprehension and skill in the language/tool of choice you’re interviewing for.
