Another possibility: Perhaps one of y'all ended up bringing up the topic of cereals (and those cereals in specific) because of a new ad campaign one of y'all saw.
You hadn't seen it yet, but lo and behold, you did later.
You hadn't seen it yet, but lo and behold, you did later.
Y’all is the Southern second-person plural, a contraction of ‘you all’ (you, second-person; all, suggesting more than one)
all y'all by comparison is generally agreed to mean everyone in the group and can be a replacement for the phrase every last one of you