>  sure we could but I don't see why you would consider it questionable to leave a method open for testing only, since it's not on the API.

IMO, it does not matter whether it is on the API or not. It is bad practice. 

> You really think it's worth our time?

I would have done it

> Also leaving it public allows - as a last resort - for other users to cast to the implementation.

But is this not the very thing we want to avoid?

>  but in this case TBH I'm not even sure why the method existed at all, which makes me uneasy on removing it.

Why? If you think it should be removed, it should be removed and make a class interna. Where is your unease coming from?