The backup endorsement is one of the cheapest riders on a homeowners policy and one of the most commonly skipped. Most homeowners who add it carry $5,000 to $25,000 of coverage - enough for a typical basement backup.
Sewage backup is Category 3 (black) water, so cleanup is a biohazard job: PPE, extraction, removal of contaminated porous material, and antimicrobial treatment. That documented scope is exactly what the endorsement pays against.
If you are unsure whether you have the endorsement, look at your declarations page or call your agent before you need it. Adding it after a backup does nothing for that loss.