This is such a marketing ploy. As you know, Morrissey's army of devoted fans is legendary, so why not bring some of them to Styles? I suppose the mechanism should work like this: a Morrissey fan sees the image of his idol on another artist's T-shirt and, voila! – he already at a subconscious level begins to "respect" a star associate (even if he is not familiar with his work). And then everything will happen by itself.
Moreover, it may not be so important for PR agents whether Harry has a strong increase in fans or not, because the main thing is views and the number of downloads.