There are some reports on German websites (here and here) that purport to associate Herrgett's martyrdom with some ties to Müntzer and the Anabaptist movement. Even Philip Schaff seems to follow this notion. I suspect these are strained attempts and speculative, and probably at best would reflect a confusion on Herrgett's part. And so I will take C.G. Hofmann's report, the source that Fick used for his book, as authoritative – that Herrgett was indeed martyred as a Lutheran. Both Fick and C.F.W. Walther judged this to be the case – these two did not accept all the so-called "Church History" emanating from Germany. —
Luther distinguishes false "martyrs" from the true ones in his day:
The factious spirits, Anabaptists, etc, are therefore the most obstinate in that they believe themselves to be vain martyrs, where they are not permitted to have their raging and raving. (StL ed., vol. 10, para. # 76, col. 899, ; not in Am. Ed.)
(tr. by BackToLuther)
"The Lutheran books they found from him were publicly burned in the market.."– Herrgett died as a Lutheran. — In the next Part 15 is the account of Nicolaus of Antwerp.