which syrup? rich simple syrup (2:1) is stable at room temperature, no bacteria you normally find survive in that much sugar. I keep 1:1 simple syrup in the fridge, that way it also last essentially forever. in things like grenadine some people add a bit of vodka to make it last longer. no idea about the video you’re looking for.
Adding liquor to a syrup to extend the lifespan has mostly been debunked. Unless you’re hitting 20%+, it just effectively lowers the sugar content without measurably increasing the alcohol content to the point of inhibiting pathogen growth.