You would think that, but you’d be wrong. Old inactive accounts get purchased and sold to bot networks all the time.

Not to defend Linkedin, but it is always possible that new measures (filters) are devised by people who don't consider all the possible parameters.

In other words, some less-thinking Linkedin employee decides to do a thing without adequate consideration of all the information available (such as length of time of membership). Maybe Linkedin let some moron apply rules, or Linkedin is grabbing data for marketing/sales purposes, or there's an explanation we haven't realized yet.

Aged Reddit accounts have a decent market. LinkedIn could easily be the same.