First off - it wasn't a Garter Snake (they come in MANY different color patterns - not just "striped"). They are strictly fish, amphibian, & insect eaters - no warm-blooded animals (or eggs). And they're groundwelling - don't care for trees/shrubs.
I recently found a good sized garter snake in my barn and moved him back out to a spot that will not freak out Mr. Trub (heck, I do not do spiders, he does not do snakes). I then did a bunch of research and the research I did does not agree with your post about them only eating fish, insects. What I read says they will eat rodents and eggs if they find them in a size they can ingest.
Am wondering if Bacardi just got confused (hey, it happens to me all the time, I often don't know what month it is and yesterday I forgot what state I live in, sigh...). Every herp key I have ever used indicates examining the stripe margins to identify garter snakes and to distinguish them from ribbon snakes. Perhaps s/he is thinking of a different species group by accident.