If they had multi-tenant databases for SaaS it would mean either the self-hosted jira instances also had the same multi-tenant database schema or they'd have to maintain two almost entirely different data access layers for cloud vs. on-prem. Since their cloud offering came from a historically on-prem codebase, I would expect the easiest way to offer cloud stuff is to do a DB per tenant. Otherwise there would a shit-ton of new code that only applies for cloud stuff....