You sir are correct! Well, the Photos app on Mac has that option, so I think it is likely you can do that in the Photo app on an iPhone as well. I would checkmating but I like doing graphics stuff on a large monitor (34" Ultrawide connected to my Mac Pro).while I havn't done so myself, I am pretty surer you can do it right on the iPhone in the Photos app (Edit) ...
So I have 2 good apps to use for B&W photo conversion.