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.
Best Answer
Dash is normally an action. The Cunning Action rogue class feature says (in part):
Cunning Action specifically lets you use a bonus action to Dash. In general, a bonus action couldn't be used this way, but Cunning Action lets you do it. Cunning Action doesn't prevent you from using your action to Dash; it only gives you additional ways to use your bonus action.
How does this work? As an action, the rogue uses Dash because this is normally how Dash works; any character can Dash as an action. Later, they use their bonus action to Dash, because Cunning Action allows them to.