The Clintons still have classified material. I bet you didn't know that part, did you? We don't go after ex-presidents for classified material. The reason why is that the president can basically declassify anything at will anyways.
Of course, being allowed to declassify anything at will isn't the same as being allowed to take anything at will. However, either way, I highly suspect that this isn't about the classified documents, since as you correctly point out, the Clintons mishandled a crapload of classified documents and information. As we know, the FBI didn't give a ****. It is about gaining access to Mar-a-Lago to look for evidence that is damaging in bringing a criminal indictment related to January 6. The warrant to investigate the classified documents charge (which may technically have merit) is a ruse to look for evidence pertaining to a January 6 charge (which almost surely does not have merit).