While it is possible this was tone mapped, the tell tale signs of it are definitely not present: there's no haloing around the mountains, the clouds aren't darker than the foreground grass, the reflection isn't significantly lighter than the sky, and there's still plenty of contrast. If this was tone mapped, it was done exceptionally well.

There may be some photoshop composite work done here because the tone of the sky and the lake is too even for it to be done in one exposure and I see little to no evidence of a graduated neutral density filter having been used (it could have easily been removed and there is a slight hint of a darkened line along the very top of the mountains in the background), if you look closely at the reflection you can see all sorts of imperfections in the water - bubbles, crap floating on the surface, etc - that works in opposition to the sky just being mirrored onto the ground. The perspective of the shore being reflected is also indicative that the reflection is legit.

There's definitely too much sharpening and the contrast is a little bit strong (too much, rather than too little which is what usually happens with tone mapped images), but the overall tone of the image doesn't suggest it would be a difficult shot to capture in one frame, there's no source of excessively bright highlights and the color is reasonably believable (probably pushed a little bit, but not to a gaudy, unrealistic level). I'm going to say this is either a legit picture, or the photographer is really, really good at masking their HDR work. Source: 12+ years experience as a landscape photographer.