Unborn prey

Peace and blessings, Rabbi Michi.
I'm planning a trip abroad and trying to figure out what kind of bread I can eat there, and I saw that in France they say you can buy a fat-free French baguette that is accepted as kosher in terms of products and relies on the Flatbread Permit.
My question is why is there no problem with the fact that the bread was apparently baked in a teref oven (with the non-kosher breads)? I heard that they base it on the fact that it was not a new-born teref. How can such a thing be assumed?
Assuming that the permission is based on this, the oven is an unborn prey, is it not when a Gentile bakes/cooks not specifically for me that there is no penalty imposed by Chazal for cooking in a vessel that is an unborn prey to begin with? What is the source of this?
Thank you very much!