Sony's camera can pull that off because its much larger sensor has pixels with an 8.4 micron pitch, while the GH5s pixels are 4.5 microns -- nearly half. That brings up another key advantage of the A7S II: The full-frame sensor is a big draw for videographers looking for more bokeh and narrower depths of field. It's not just an arty trick with video, but an important tool for, say, shifting focus from one subject to another.
At the same time, sometimes you need a deep depth of field for video, and a smaller sensor actually helps there. On the A7S II, you might need an aperture setting of f/4.0 to keep items in a scene in focus, compared to f/2.8 on the Panasonic. In that case, the Sony loses some of its low-light advantage.
A few other things: Sony's A7S II has a bad reputation for rolling shutter, aka jello effect, especially at 4K. Panasonic claims the GH5s reduces rolling shutter by 25 percent compared to the GH5, which was already better than the A7S II. And again, if you need to shoot slow motion or high frame rates, the Panasonic is the only game in town for 4K at 60fps and can manage the feat in DCI 4K, which Sony doesn't even support.
While this review is video focused, I have a few observations about taking photos with the GH5s. 10.2 is not a lot of megapixels; four by three photos are 3,680 x 2,760 in size, which seems comically small compared to, say, the 42.4-megapixel Sony A7R III. As such, it simply won't do for landscape, portrait and other photographers.
On the other hand, my photos were plenty sharp (using Panasonic's excellent Leica 12-60 f/2.8-4.0 lens), and with 14-bit RAW, you can dig into shadows and highlights to recover detail. The contrast-detect AF, while still inferior to phase-detect AF from Sony and especially Canon, has never worked better than on the GH5s, I found. And when using center-weighted metering, most of my shots had perfect exposure.
In sum, Panasonic has really done a great job targeting the video crowd with the GH5s. By switching back to a multi-active sensor, it squeezed the most out of the micro four thirds format, which should give pause to folks considering an APS-C model like the Sony A6500 or Fujifilm's new X-H1. It also blows away nearly every camera, save Sony's A7S II, in low-light performance. And even where it falls down there, the GH5s' excellent video codec and high bit rates can save the day.
But will it convince Sony or even Canon 5D mark IV video shooters to switch? I'm not so sure about that. People who like large sensors would very reluctantly go down to one half the size in exchange for a more robust codec, in my opinion. And the GH5s is still limited to contrast detect focus. It seems to work a bit better for video than the GH5, but Canon's dual-pixel, phase-detect autofocus is the gold standard -- it's much faster and more reliable.
It'll also be tough to get GH5 shooters on board. Many of those folks are not pleased about the lack of IBS on the GH5s, especially the vlogging crowd. At the same time, the GH5 is a great camera for general photography, but a 10.2-megapixel mirrorless that costs $400 more is a pretty tough sell, even with the GH5s' new 14-bit RAW mode.
As someone who does a fair amount of video work, I applaud Panasonic for building the GH5s and would recommend it for pro or keen amateur video shooters over Sony's A7S II. At the same time, if you don't depend on video for your livelihood, I wouldn't advise spending $2,400 on it (nor would i recommend the A7S II). If you do both video and photography equally, then your best option is probably, yes, the older GH5 or, if you have more cash and want a full-frame sensor, the A7R III or Canon 5D Mark IV.