I'm only suspicious if they don't simultaneously and eagerly show me their Github so that I can see what they've accomplished.
Sometimes this industry is a lot like the "finance" industry: People struggling for credibility talk about it constantly, everywhere. They flex and bloviate and look for surrogates for accomplishments wherever they can be found. Peacocking on github, writing yet another tutorial on what tokens are and how embeddings work, etc.
That obviously doesn't mean in all cases, and there are loads of stellar talents that have a strong online presence. But by itself it is close to meaningless, and my experience is that it is usually a negative indicator.
But if you mostly mean in the sense that they don't have a fancy GitHub profile with a ton of followers... I agree, that does seem to be the case.
LinkedIn on the other hand... I sincerely can't imagine networking on LinkedIn being a fraction as useful as networking... Well, at work. For anyone that has a decent enough resume LinkedIn is basically only useful if you want a gutter trash news feed and to be spammed by dubious job offers 24/7. Could be wrong, but I'm starting to think you should really only stoop to LinkedIn in moments of desperation...
there is an entire industry of devs who work on meaningful projects, independently or for an employer who solve amazing problems and do amazing sh*t none of which is public or will ever be public.
I have signed more NDAs in my career than I have commits in public repos :)
Someone like you is extremely experienced and skilled, and has a reputation in your industry. You started working before it was normal and trivial to build stuff in public. Such activities were even frowned upon if I recall correctly (a friend got fired merely for emailing a dll to a friend to debug a crash; another was reprimanded for working on his own projects after hours at the office, even though he never intended to ever sell them).
That you have a reputation means posting work publicly would be of little to no value to your reputation. All the people who need to know you already do or know someone reputable who does.