Maybe they only stock nobivac due to contracts. If it’s a practiise with a few vets, it might be worthwhile to ask to clarify it with the head vet. Also a receptionist will only tell you the rules as they know it.They are overworked, grumbled at, and not allowed to make decisions on anything . My son was late for boosters and receptionist insisted he need to start all over, but the vet was sensible and just did booster.
Google says, like you, that nobivac and Canigen are compatible.