I think it means it was targeting older model iPhones that are updated to the newest available software from Apple but aren’t supported by the newest versions of iOS. Apple generally releases security updates for these older versions for major vulnerabilities when they know about them, but as stated this was a zero-day so they had not had the opportunity to patch it before the attack.

Security fixes are backported with some frequency, but new mitigations are usually not. If a bug is made inexploitable by a mitigation in the latest OS, older OSes that have the same vulnerable code can probably be exploited, even though they still get security updates.