Given the context of the lecture, it's a bit of an odd choice to use the word guess in this sentence. He's trying to emphasize that the American Sign Language gesture for "pasteurized milk" is rather funny because it's literally symbolized as "past your eyes milk." A better choice of word, and what I would surmise he was likely going for, could be
"You'd better believe the folks at that Galaudet College went out
and celebrated the rest of the day when somebody came up with that
"You'd better believe..." is an idiom/expression used for telling someone that something is definitely true and that they should take it seriously. In this case, the lecturer is jokingly saying that the gesture looks ridiculous, but in fact, it is a real gesture that actually means "pasteurized milk".