In the first place, I'd urgently recommend getting a more recent translation! The fact that a translation is over three hundred years old is no argument for accuracy and legibility -- quite the reverse (and especially if it adopts quirks like transliterating graecised Roman names back into the Roman alphabet from Greek, instead of just using the original Roman names -- leading to "Sylla" in place of the correct spelling "Sulla").

But this problem, at least, is not the translator's fault but the web publisher's. It appears that "ACE" is a rare variant of "CE"/"AD", and that it does usually stand for "after common era". I agree with you that it is an unintuitive usage.

It is also wrong. At Sulla 21.8 Plutarch mentions that it has been "nearly 200 years" since the battle of Orchomenos in 86 BCE. Exactly 200 years after would be 114 CE (assuming that Plutarch followed usual practice and counted inclusively); C. P. Jones takes "almost 200 years" to mean "190 to 200" years, and so dates the Lives of Lysander and Sulla to between 104 and 114 CE ("Toward a chronology of Plutarch's works", Journal of Roman Studies 56 (1966) 61-74, at 69). This kind of reference is typical for the kind of evidence that gets used to date texts like this. According to the rest of Jones' work, this puts Sulla about a third to a half of the way through the writing of the Lives, but within the last twelve years of Plutarch's writing career (none of his works can be dated later than 116; this terminus is based on references in his works to Quintus Sosius Senecio, and based on the fact that Sosius seems to have died before 116).