It is probably nonsense if the whole point of "establishing an online presence" is to be marketable.
It is not nonsense if you are genuinely interested in participating in those online activities (or happen to enjoy contributing to some open-source projects).
And if you do participate, you will leave an online trail that will help your potential interviewer to get to know you better (what technologies you actually know and how deeply; what your coding style is; how good you are at writing code in general; that sort of thing).
Of course, much of that can be established during an interview. But every additional data point helps to better assess the candidate.
So the point is, it’s not that they should, but it helps if they do.
It is not nonsense if you are genuinely interested in participating in those online activities (or happen to enjoy contributing to some open-source projects).
And if you do participate, you will leave an online trail that will help your potential interviewer to get to know you better (what technologies you actually know and how deeply; what your coding style is; how good you are at writing code in general; that sort of thing).
Of course, much of that can be established during an interview. But every additional data point helps to better assess the candidate.
So the point is, it’s not that they should, but it helps if they do.