On further reflection, I remember creating that account with no connections while investigating what was going on. I tried to log in from the email address that was receiving the emails but couldn't and assumed that the account had been cancelled for inactivity. I "reopened" the account (in reality creating a new one) on accident. So I retract any statements about them accidentally creating accounts, this was a while back and I had forgotten that portion.
But to be clear, that wohle scenario happened because LinkedIn started sending info to that (unaffiliated) email address unexpectedly, and I assumed if I was getting LinkedIn email there, that must be my account email...