Update: Okay, so I created a flowchart in one instance, then shared the link, and opened it in another tab. Edited the flowchart in one tab, but it didn't change in the other tab. This is imho not how a cloud OS should work.
But the container technology seems quite cool.
Sandstorm does not dictate what tech stack you use -- it supports any server stack that runs on Linux. Supporting real-time collaboration is normally the job of your tech stack. If you use Meteor, for example, you'll pretty much have it by default.
Sandstorm does make real-time collaboration easier to operate at scale through fine-grained containerization: each "document" runs in a separate container as a separate instance of the app. Sandstorm ensures that there is never more than one instance running for a particular document, so you don't have to worry about concurrent writes coming from another machine. Scalability is achieved by distributing the documents across machines.
Unless everyone wants to edit the same doc at the same time.