I've found two chains of Twitter posts by Jeremy Crawford that I can't seem to make sense of.
According to a Crawford tweet from August 2018:
Actions and bonus actions aren't interchangeable.
However, this series of Crawford tweets in March 2016 says that a rogue can Dash twice in one turn if they Dash as a bonus action.
So how can a rogue use both their action and bonus action to Dash with Cunning Action? It is specified in Cunning Action that the Dash can only be used with a bonus action.