That would be a hot news story and well worth reporting, but I've done some work of my own (3 minutes with Google) and I don't think this story is legit.
Update: And now Xiaomi has denied the accuracy of the initial report.
After a few minutes of sleuthing, I found a product listing on Banggood.com for a Xiaomi laptop which uses the same stock photos and has the same specs as mentioned in the GizmoChina article. That listing shows the "Xiaomi Ultrabook" as being up for pre-order with a price of $710, but I don't think it's real.
Here's the photo from Banggood (aside from the lack of a watermark, it is identical to the photo found by GizmoChina this morning):
Looking at the questions asked by Banggood customers, we can see that the retailer has admitted to not having any actual information on the laptop. They do insist at one point that Xiaomi has said that it's coming, but this listing has been up since at least August 2014 - with no formal announcement from Xiaomi.
While it is entirely possible that Xiaomi is working on a laptop, I think this report is a fake.
Aside from the fact that this listing has been up since August, the other way you can tell that it's a fake is that the images used actually belong to a reference design laptop. As Liliputing pointed out, someone (in this case Banggood) simply photoshopped Xiaomi logos onto existing pictures and called it good.
And while I would normally simply ignore a fake story, I thought it might be useful today to instead debunk it.