Signed commits still let you falsify your own work after the fact. You need an external trusted service, a digital notary essentially, that would provide a signed record that you submitted a commit on a certain date with certain set of ancestor commits. Each time you added an entry, you would submit the hashes of all the entries in the book to the digital notary and get a signed record that you submitted those commits on that date. Inability to produce this record would be similar to having erasures or pages torn out in a lab book.