We have been in the process of working through porting the remaining Document Library actions to Aikau, and the Edit Online action is one of these. This epic lists all the action issues, this would be the most relevant issue.
The Aikau team is a component (rather than feature) team and delivers requirement made by other feature teams. Because no feature team have required the edit online action we haven't implemented it yet.
We did actually take a look at this action during a couple of sprints, but it remains unfinished. If there is sufficient demand from the community to implement this action (before there is demand from within the Alfresco feature teams) then we will implement this.