If we take the numbers out of it-
They cut extremely short their participation in their granddaughter's birthday party, including skipping out on presumably expensive activities that were planned with them in mind, so they could spend the day at the anniversary bbq of FIL's brother whom he wasn't close to.
So, uncle- and aunt-in-law get to have the ILs for most of the day despite short notice and a further relationship, while granddaughter gets to suck it up despite having had their promise from months ahead.
I've been that grandchild where they're gung-ho to spend time with you, until there's another offer, or they actually get the chance. It hurts, it seriously damages the relationship, and you notice. My parents managed to shield me for most of my life, mostly due to physical distance and my grandmothers' actions being normal to me ("oh, grandma just dropped the animals with me and left, big shocker"), but after a point you grow and realize that other people are more important to them.
OP, I think you're doing fine. If anything, you're handling things the way my parents did, and that worked pretty darn well.