The 13th Amendment dealt with current slaves. Slavery was something that they were dealing with each and every single day. Any contract or arrangement that currently conferring ownership of one human being to another was now nullified by the Constitution. It didn't matter how or why that arrangement/contract came into being. It didn't seek to address those individuals who were "improperly" enslaved. The 13th Amendment simply sought to abolish the entire institution of slavery altogether. From the moment it was enshrined slavery ceased to exist as a trade/practice. Any and all people who were planning on acquiring slaves in the future, for whatever reason, would never be allowed to do so. Period.
The other reason that your analogy is bogus is that the 13th Amendment was a not a bill of legislation but a bona fide CONSTITUTIONAL AMENDMENT passed by two thirds of both houses and then ratified by the States.
No one is saying that 924c needs to be abolished. There are individuals that warrant prosecution under this statute. The issue is that this statute needs to be used as the legislators intended. It was misapplied in this particular case.
Who cares about the 13th, if you think it was a bad anology fine, it doesn’t change my contention that ex post facto applies to perps, not victims. It applies to people being prosecuted for crimes prior to a law being enacted, not freeing victims of laws badly written.