It’s not a mis-print, anyone who has Jurgens 3-volume set is aware of the 80 AD date. Madrid is going by the Jurgens date for the letter, while virtually all scholarly lists of the Popes put Clement’s reign in the 90s (so Madrid follows that as well). But Jurgens would suggest an earlier reign for Clement. Fr. Jurgens explains his dating:
"The various early lists of the Bishops of Rome make Clement either the first, second, or third successor of St. Peter. The better evidence and that generally accepted would have him Peter’s third successor, following after Anencletus (also called Cletus)…
“The traditional dates of Clement’s pontificate, AD 92 to AD 101, are unworthy of credence. Believing that there is good evidence for dating his sole extant authentic writing c. AD 80, a work clearly written while he was Bishop of Rome, the present author dates Clement’s pontificate accordingly…In regard to the date of composition, there is an almost universal acceptance, for no good reason, of the date 96/98 AD. This dating is based upon an acceptance of the years 92-101 AD as constituting the term of Clement’s pontificate – dates which otherwise are taken seriously by no one! – and upon the opening words of the body of the letter…which are taken are referring to the persecution under Domitian, in order to fit the obscurely-alluded-to events into the period 92-101 AD. However, that there was a persecution under Domitian is a supposed fact which rests upon very slim evidence, and is itself scarcely more than a conjecture.” (Jurgens, Faith of the Early Fathers, volume 1, page 6-7)
Jurgens attempts to present “better internal evidence” for the 80 AD dating. Madrid should have adjusted his dating for Clement’s reign if he wanted to follow Jurgens in his dating. Madrid seems to follow the “standard” dating for Clement’s papacy later in his book. And most date the letter to about 95 AD as well (e.g. JND Kelly, Oxford Dictionary of Popes).