I don't know how the dictionary definition of "venom" would read, but to me for it to be a venom it would require a specialised delivery system (fangs, grooved teeth, sting etc). Also, surely the purpose of a venom is to subdue prey prior to ingesting it. In the case of Garters which just seize and swallow, the "venom" is clearly not serving this purpose.
The anti-coagulant however would ensure the puncture wounds caused by swallowing would remain open to allow in ingress of digestive fluids. So it's an aid to digestion.
I can't comment on the neurotoxins that may be present, I haven't read that. However the book Stumpfbandnattern is in the post on it's way to me even as I type! Problem....Ich sprecken keine Deutch...Obviously!!!
