I can see why you're in two minds about it Kammer. It looks oh so possible, yet not quite defined enough to be sure. In a similar way to the markings at Castlerigg, the Goggleby stone and Moor Divock, all of which have been said to bear markings, none of which are universally accepted as definite.

Indistinct maybe-markings seem to be a feature of ancient Cumbrian monuments. By comparison, whilst it's hardly Long Meg, (which is an unrepresentative example as it's sedimentary stone, and as such lends itself much more easily to well-defined marking) that Swinside 'axe' is fairly well defined. Certaintly when compared to the elusive Castlerigg spiral.

I'll hedge my bets and go with 'artificially enhanced natural feature', which covers a multitude of sins.