The ending works on two levels: the eruption fueled by the crystals pushes the main asteroid away from Earth and saves the world, and the accidental shard that lands in a hot spring returns youth to the Geotopians. That second beat quietly undoes Sid's mistake, since he is the one who destroyed their crystals in the first place.
The film then closes at the wedding of Peaches and Julian, with Diego and Shira honored by young animals and Brooke back with Sid. In my opinion, the wedding is the real ending, because it pays off the family thread that Continental Drift began when Peaches was a teenager clashing with her father. The asteroid plot is the spectacle, while Manny letting go of his daughter is the story.
Perhaps the neatest detail is who finishes the job: Julian helps Manny push in the final crystal, so the future son in law is part of the rescue rather than a bystander. Buck persuading Gavin's flying dinosaurs to rejoin the effort also matters, since it turns their selfish escape plan into a team effort.
That wraps up the family storylines and leaves the herd together and settled. It could be argued that this is why the film feels like a farewell to the theatrical run, though the saga did continue on Disney Plus with ice age buck wild, and a sixth theatrical film, Ice Age: Boiling Point, is set for February 5, 2027. As for whether Scrat finally ate his acorn, published plot summaries of the ending center on the eruption and the wedding and do not have him finishing it. His chase is what starts the disaster.