No, I find that pretty easy to imagine.
I can think of plenty of developers working in non-OSS (either internal/nonlicensed or proprietary licensed) shops who, even if they have and work with OSS dependencies, and even if they might hack on them for the sake of the projects they work on, have never, by (or at least as a result of the requirements of) employer policy, contributed a thing back.
Of course, that's because I've spent most of my career in that kind of environment, which is why I've got very little on my GitHub none of which has a direct relationship to my paid work.
We work in GitHub, and of those who use their personal profile for work, all of them have "empty" profiles and no meaningful OSS contributions. They spend their free time with their family and in other hobbies.
People who hack on OSS for fun are usually good developers, but good developers rarely hack on OSS for fun especially once they get older.
I meant, using FLOSS libraries for the actual job, during normal work hours. Depends on the industry, of course, I guess.
I think there's many more working on proprietary software, we just naturally don't hear about it due to its nature. Open source is prominent only because it's public.
E.g. I open Slack (a random pick - it's open on my desktop right now), a proprietary software app. And in the "about" section there's a huge list of FLOSS libraries they use: https://slack.com/libs/desktop
Moreover, I'd say, every website out there (and web is a giant niche) uses lots of FLOSS components.
And for GitHub contributions, I meant that once in many years there happens to be a library that's out of date but otherwise useful, or lacks some nice feature.
On the other hand, it's hard to imagine someone senior enough (in terms of experience, not years) who haven't ever hacked a FLOSS dependency to do something it originally didn't.
Of course, it could be that the source wasn't on GitHub, or that author did effort to persuade upstream into accepting their changes and then repository was removed, or that the code is in the company's internal systems (which can be a good idea for various reasons). However, I'd say that most typical scenario is "you see it, you find it on GitHub, you fork it, patch it and leave it there - just in case". I have a bunch of those, and I'm just archiving those repos when I recognize they're not relevant anymore. It's not like I'm paying for those - and having an ancient fork had helped me once, when someone from my old place had a problem and asked me if I still remember how things worked there.
I mean, many (most?) of software engineers are directly working with free software every day. And, sadly, world isn't that perfect this software provides everything one might need without any tinkering. :) So, in a certain sense, "no time for open source" is a little bit questionable.
But, yeah, there are valid reasons why a good engineer may not even have GitHub account at all. Unless you're drowning in a torrent of great-looking resumes (and you're not GitHub Inc.), no harm in not making GH account a mandatory requirement.