The thing is, it's specifically called out in o5e, at least, that you cannot use a bonus action as an action, which would suggest that it is not, in fact, an action. I order to use a bonus action, you must have a bonus action available, not an action. So I would argue the opposite, that it's a separate thing and it should be made clear if surprise interacts with it. This is also relevant for the paralyzed condition (i think?) which doesn't specify that you can't take bonus actions, which seems like an oversight.