From section S7.3.5.1 of the AVMA Guide:
Carbon dioxide—Carbon dioxide may be considered for euthanasia of amphibians and reptiles if alternate methods are not practical and where the limitations of this method are understood and addressed. Due to the potential lack of response to this method by many species and the requirement for a prolonged exposure time, other methods are preferable. Death by CO2 must be verified, and preferably, assured by application of a secondary lethal procedure.
While it clearly states that other methods are preferable, it does list CO2 as something to be considered. No where does it refer to this method as inhumane.