>How do you know they had no problem socializing for long periods? Did you ask them
Yes of course. I think pretty much every self declared introvert Ive asked about this had 1-5 people for whom spending time with them didnt "drain" them - e.g. best friend, parent, spouse, etc.
When they described what type of situations drained them that they enjoyed it would generally be something like going to a party where they didnt know anyone.