It's an interview, so a more accurate piece of feedback would have been "you couldn't convince us you have practical experience programming."
So maybe they had a question like, how would you solve those problem, and you said, here's the solution, but they really wanted you to say, there are three potential approaches that i would use, these are the things i evaluate to decide which approach is likely best, these are the war stories that I've had that led me to refine my thinking to this point.
Yeah I guess but how much open source do they want? If you've already got a hundred thousand lines and it isn't fixing what they're after then suggesting something else would be more helpful. But I guess their interview method works for them.
They're not looking for open source per se, they're looking for you to demonstrate range/depth of experience. If you haven't been able to pick it up from your job, because they're too limited in the types of projects and assignments you can work on, then they're hoping that you can get it yourself from open source.
Now, you may already have the abilities that they want you to have, then what you lack is the ability to show so in the interview.
edit: Just to clarify, in technical interviews, they often don't even look at your resume, they just ask you the questions and judge you on how you answer them. Stating your years of experience or lines of code written have no bearing.
I don't know if we're talking past each other, but if you've already done tons of open source, and it hasn't gotten you the skills they want, then is just repeating 'do yet more open source' useful advice? That's what I mean. If someone had done tons of open source and it wasn't coming through in terms of skills I'd give them advice like 'get a friend to help you practice interviewing'.
Ok gotcha. I think you're asking a bit too much from them to give you personally tailored advice. Your interviewers most likely didn't even read your resume. Most companies will just send you pre-formatted email "sorry you weren't a fit".
If you're trying to parse their message "we're not sure you have enough practical experience programming - maybe try working on an open source project" then I would interpret it as I stated above - they didn't think you demonstrated the depth of experience that they were looking for, and were hoping that working on more types of projects, which you might not get at your job, but you might get by working on a whole bunch of different open source projects, might get you.
So maybe they had a question like, how would you solve those problem, and you said, here's the solution, but they really wanted you to say, there are three potential approaches that i would use, these are the things i evaluate to decide which approach is likely best, these are the war stories that I've had that led me to refine my thinking to this point.