It's ridiculous because a past civilization wouldn't have spread iron-60 all over the planet the way a supernova would have.
But a fairly advanced civilization could have existed in a fairly local part of the planet that we don't much explore (think of the Sahara desert) and so haven't found the remains of, or it could be buried or under quite a bit of water (if it developed during a glacial period).
But a fairly advanced civilization could have existed in a fairly local part of the planet that we don't much explore (think of the Sahara desert) and so haven't found the remains of, or it could be buried or under quite a bit of water (if it developed during a glacial period).