Well it was in the introduction of first bible I was given years ago - but that was 40 years ago and I can't find it anymore to give you more details. I've heard others say it - they might be a group repeating each other, but still that is two different groups.
What is the generally accepted meaning?
It could be affection and spiritual relationship. Or it could be a biological son but this Mark is not John Mark who is likely to be the evangelist. https://overviewbible.com/john-mark/
I guess at this point, I'm legitimately asking who holds this view such that one can say "It is thought that . . ."