Probably they’re thinking of the Peasants’ War and the Münster Rebellion (the latter of which, by the way, was attempting to set up a theocracy.) The leaders of each movement, which happened within Protestant territory in Luthor’s own lifetime, had some points of contact with Anabaptists, but aren’t necessarily regarded as being the same. Depends on which theory of Protestant development one adheres to.
It isn’t simply a case of Luthor oppressing them, though; arguably they took up arms first against ruling princes in the area, although just as arguably they felt like they had been provoked into doing it. Luthor had to agree with putting them down. I don’t gather he was happy about it. (His successor Zwingli, on the other hand, definitely persecuted Anabaptists explicitly.)
It’s also important to keep in mind that, by the understanding of practically everyone at the time (including the Anabaptists), only baptized people could go to heaven; yet the Anabaptists were, despite their name, not just insisting on a second believer’s baptism at an age of responsible consent, but were outright forbidding babies to be baptized at all. So to everyone else, the Anabaptists were practicing something actually worse than murdering children.
Consequently, everyone (even the chief Protestant Reformers) decided that the Emperors Theodosius I and Justinian I were both correct to order that “Donatists” (which the Anabaptists were somewhat identified with) should and would be executed.
It was a nasty piece of business–and while it can plausibly be laid at the feet of the belief that there must be some kind of hopelessness which must be avoided at all costs (so also at the cost of any act which would otherwise be decried as barbaric injustice), there’s a cautionary moral for universalists in there, too, or for some of us anyway. (Not the ultra-u’s I guess.) Because after all, when they were torturing Anabaptists (and each other), they were trying to ‘lead sinners to repentance’ in an explicitly purgatorial fashion.
Hopefully, God does a more nuanced and finely-tuned job of that (unless ultra-u is true.)