Yes that's right. It's just one of the pleasantly convenient anachronisms that allow the militants a relatively wider choice of armours. All in all, a 30 year anachronism isn't really a major issue for sagas that could well last over 100 years.
Call it a Targe shield or (even better<) a comet shield. The "heater" or Ars Magica is likely to be a misstranslation and represent (more accurately for the period) the comet-shape shields carried by the cavalry at the time of Ars Magica and before.
The more I read of medieval history, the more I realize that any single source (Wikipedia, mainstream book, scholarly article) cannot be trusted when it comes to exact dates for things like weapons, architecture styles, or clothing styles.
Professional historians make their living by contradicting one another. For any source that says something appeared around date Y, someone else will say it appeared at earlier date X.
I also think a heater is so little different from the earlier "kite" shield (an ambiguous term I've seen applied to two different shield designs) that they should have the same game statistics.