Not only was the law an unconstitutional ex post facto law, it was also an unconstitutional bill of attainder because the law was passed specifically to target Donald Trump
It’s not ex post facto because it’s not a criminal law, and it’s not unconstitutional because the statute of limitation is an important but essentially random determination by the Legislature that it has the power to change. Run your argument by the Boy Scouts of America that got bankrupted by having the statute reopened in a number of states, for many very old claims that could not actually be documented, and years after the BSA enacted policies to eliminate abuse. No program is perfect, but that’s why the cliche of abuse in recent years typically refers to the Catholic Church, not the BSA