My bet is that another scaling step is causing it; and HiDPI mode is "native", so no scaling.
You can see this in the comparison of the two screenshots; the first one looks like an antialising filter was applied to all the pixels and halved the resolution (so a 2x2 block of 4 pixels turns into one averaged "huge" pixel), the second one doesn't.
macOS has two rendering modes: Normal (1x) and HiDPI / Retina (2x). For this reason, most apps have two different sets of assets like icons and other images shown in the UI.
After rendering the content either at 1x or 2x, the picture is scaled to the "real" resolution of the display. This allows Apple to support a large number of "scaled" resolutions, but the apps only have to support two modes (1x or 2x).
You can see that this happens if you have two displays attached to your Mac, one with Retina and one without. When you drag a window from the Non-Retina-Display to the Retina-Display it will be pixelated at first (it was drawin in 1x mode), until it's completely on the new display, then it will redraw in 2x mode and become sharp.
Normally 1x mode is faster. Try opening an app that shows a lot of data (eg. a large spreadsheet) and scroll quickly on a 1080p display vs. on a 4K display. You'll should see that the scrolling is much smoother on 1x than on 2x, since the 2x display needs to draw four times as many pixels.
So this is clearly a bug, and not because of some additional scaling from 2x->1x. As this blog post shows, it's faster when downscaling from 2x.
(Also, the pictures in the blog look like they have been resized for the blog, so I wouldn't read too much into their blurriness)
That's not always the case. It can be, depending on the resolution selected (in this case it's "default for display" so no idea).
HiDPI just means 2x rendering. But it still might be scaled to the resolution you've chosen for your display.
On my MacBook Pro 13", there are four scale choices from "larger text" to "more space". The first option upscales (slightly blurry), the second option displays the 2x directly ("natively"), and the third and fourth options downscale.
They're all still HiDPI though. In contrast, when I plug into my 1080p projector, it goes back to 1x ("LoDPI"?) rendering.