“Disks are usually the slowest part of a database so limiting page fetches can have a huge performance win.”
SQLite packs rows into 4KB pages and arranges them in a b-tree, so finding one record in a huge file takes a handful of reads. That design is from the early 2000s and still beats half the hosted databases people pay for. Fly.io is explaining it because it sells SQLite hosting now. The explanation is still good.