User blog comment:IamJakuhoRaikoben/Chapter 482: Vigor, Review./@comment-24102956-20160419153757/@comment-7418318-20160419222046

Why would she have to be a Spriggan 12 for Neinhart to know her?

Also the timing doesn't seem to work on that theory. If Eileen left the Spriggans prior to Erza's birth, that'd be almost or maybe even more than 30 year ago depending on how quickly she got pregnant (Erza was 19 in 784, plus the two timekips makes 27 years minimum).

Even if Neinhart had joined the Alvarez forces in some capacity (not necessarily a Spriggan yet) at 15, that'd would make him 42 or more years old and he certainly doesn't look like that.