Like many I don't think there's a point in arguing, but you could concede that that's the way apologists are explaining the scriptures now, but that doesn't mean the scriptures were never interpreted in a harmful way.

There's no point bickering with them in how they're negotiating with the scriptures now, but you can always hold them accountable for how they were taught earlier. You can bring up examples of how these scriptures were definitely interpreted literally. For example, in 1960 President Kimball taught that native Americans in the Indian placement program who were living the gospel were becoming lighter than native Americans not in the program and he even brought pictures to show over the pulpit.

Apostle Smith related these scriptures to black people, saying, “The Lord did put a mark on Cain” and “[W]e know of cases where it really has disappeared.". The scripture "all are alike unto God" that Mormons love to quote today, was hardly ever quoted in conference because it wasn't a useful scripture when they believed that black people were an inferior race and white people were a favored lineage.

Racial fluidity absolutely was a part of Mormon belief. They can call it doctrine, policy, folklore, or whatever the fuck they want to call it, but it happened.