There are a couple things missing from this:

1. The monitoring client does not ensure that the checkpoint was created recently, so a malicious log can conceal malicious entries from monitors by serving an old checkpoint.

2. Though the age keyserver policy is not configured this way, the post suggests you could create a policy that requires only a minority of witnesses (e.g. 3 of 10) to cosign a checkpoint. If you do this, then monitors have to get checkpoints that are cosigned by at least 8 of the 10 witnesses. Otherwise, a malicious log could present one view to relying parties that is cosigned by one set of witnesses, and a different view to monitors that is cosigned by a different set of witnesses. There is currently no mechanism specified for monitors to get these extra cosignatures, so if you go with a minority policy you'll need to invent your own stuff in order for witnessing to actually accomplish anything.