Indeed, I don't think it's 100% clear either way.
IMO, based on the Alloa highlights, Strachan just gets enough on the ball to make it a good tackle. He does then catch Rowe and, like you, I'm not 100% sure what the law is there.
Re tackling from behind, I think the only thing the laws specifically say about that is to emphasise that a tackle from behind which endangers an opponents safety should be a red. I don't think that applies here as it was more from the side and the force wasn't great enough. Could be wrong as it's a while since I've really studied them.