Rashi has Ruach Hakodesh. Every word he uttered and wrote is with divine inspiration and intervention.
Additionally, Rivka getting married at age 3 is the classical and standard explanation to cheder boys in Pre-1-A through Beis Medrash.
I should add, aside from these points, Rashi surely had a more authoritive version of Seder HaOlam — closer to its authorship than us — despite any so-called “modern research” (a/k/a rubbish).
If someone doesn’t like such due to modern 20th century sensibilities, so be it. Our Torah is eternal and needs no revisions.