I’m not good with the actual sources, but it would be good to see them here, if anyone can show them.
See Pri Megadim (Eshel Avraham) in O.C. 451:30, towards the end, where it’s clear that ordinary use of an oven doesn’t burn out the issur.
However, the bigger problem here anyhow is the potential for the (supposedly – we haven’t established this) kosher muffin being in the oven simultaneously with non kosher food. Even if they didn’t touch, see Shulchan Aruch Y.D. 108 (R’ma) that l’chatchilah you shouldn’t buy it.
Plus, here, we have no way of knowing if it did or didn’t touch any non kosher food (or non kosher tray or rack).