He seems to be serious - he even describes the judicial process that would lead to eventual executions and goes on to tell that it would guarantee that things would not be done in haste.
How effing sick does one have to be even to entertain such thoughts?
I don't know - it doesn't read to me like he's decribing a judicial procedure to be put in place. It reads like he's saying that they had such procedures in place in the Bible, so that kids weren't actually killed, so this outdated bit of morality is okay to find in a book I have to believe is relevant today.
It's worrying enough that he feels it necessary to defend, but I don't think he's advocating its introduction today.