Yes, when you render an image that's larger than your monitor's resolution and subsequently downscale it, you generally have the possibility of getting anti-aliasing from this. However, depending on the method used to downscale, the effect of the AA can vary. SSAA basically does the process of generating a large image and then downscaling it, but it's very efficient at blending pixels in comparison to Dolphin's downscaling filter. Dolphin's way of downscaling isn't optimal in this regard, so you can actually see a noticeable difference when adding AA versus just using an IR higher than your monitor's resolution.
You'll definitely see AA if you set your IR to 5x or 6x but only have a 1080p monitor, but it's not nearly as effective as dedicated AA methods, and even at high IRs downscaled to 1080p you may still see "jaggies". The AA is there, just not as strong as it could be. So in short, it's not a panacea for all of your aliased pixels. However, high IRs in combination with other AA may be worthwhile in terms of image quality versus performance. For example, you could go 4x IR on a 1080p monitor to get a slight AA effect, then clear up geometry based aliasing with MSAA (which is really cheap on hardware resources).
You'll definitely see AA if you set your IR to 5x or 6x but only have a 1080p monitor, but it's not nearly as effective as dedicated AA methods, and even at high IRs downscaled to 1080p you may still see "jaggies". The AA is there, just not as strong as it could be. So in short, it's not a panacea for all of your aliased pixels. However, high IRs in combination with other AA may be worthwhile in terms of image quality versus performance. For example, you could go 4x IR on a 1080p monitor to get a slight AA effect, then clear up geometry based aliasing with MSAA (which is really cheap on hardware resources).