It's perfectly possible for them to update map data without an iOS updates. However it's also possible for them to make some data updates only available for certain versions of iOS. e.g. they could release simple corrections to all versions and then tie major changes to the next iOS, which would allow beta testers to test before the wider public had access.
I have no idea how they actually do it, just saying this is a possibility and one of many potential models they may use for updates.