I undeleted this older topic because it relates to impersonate which is currently being looked at in Add 'stop impersonating' link to user avatar dropdown when impersonating - #41 by ted.
I noticed that the behavior in the OP is still the same as back in 2013. If you impersonate a user it updates their last seen date, which is easily reproable by looking at the admin user list. I just impersonated a user on my personal site that hadn’t been seen for a few years and they are now second in the list.
I suspect the same is true on the public user list and in who’s online if that plugin is installed.