Just discovered recently that keynote does have a collaboration function through iCloud. Was a little clunky to get going but is working great now. Not quite google docs but it does instantly sync across devices when you save
Disclosure: Was a QA Engineer at Apple on those apps/that functionality.
Ironically, because Google and Microsoft collaboration apps don't have iOS platform control, they are much more flexible in supporting multiple user identities.
There are many scenarios where information needs to be separated (projects, consultants, cross-org teams, OSS projects) and multiple identities are a proven means of separation and compliance. This needs to be possible without Enterprise MDM.
For my use, there's a "Collaborate" button. You click it and invite people. Once they click the invitation, you can watch them view/edit.