I am a fan of SMBC, but the entire explanation is wrong. The events that led to the integration of reproductive and digestive systems happened long before a terrestrial existence of vertebrates, and certainly long before hands. To get a start on a real explanation you have to go back to early bilaterals:

http://www.leeds.ac.uk/chb/lectures/anatomy9.html

As near as I can tell it was about pipe reuse. But you can't make a funny comic about that (or maybe you can?). Zach is a "bard", not a "wizard." He entertains.