He's using a pc monitor, that shouldn't have much display lag. Bandwidth has nothing to do with latency. So either his internet is set up wrong, maybe playing through a vpn, or his isp sucks. There is no way a phone outperforms a pc in latency.
From the article: (where that video is from)
On the phone, however, a totally different story emerges. While playing Destiny 2 in 60 fps on a Google Pixel 3a XL over a WiFi connection, there was almost no blurriness and barely any latency. Barring a few noticeable but quick skips during play, the images produced were fast and sharp on the phone’s 2160x1080 resolution screen.
These tests were conducted with my WiFi at home, which gave me anywhere from 45 to 55 Mbps download speeds, well above the 35 Mbps Google says should give me 60 frames-per-second movement in 4K resolution. They were also conducted on The Washington Post’s Gigabit Ethernet and WiFi service. In all cases, the phone outperformed the experience on browsers and TVs.
How was his wifi set up, did he play through a vpn. Again download speeds don't mean anything. Did he do a ping and jitter test.
Eurogamer did a real test
Yet they admit trouble as well with their internet
We're focusing on the big screen primarily because right now at least, it is the only way to access Stadia's top-end video output - ultra HD at 60 frames per second with HDR support. However, actually accessing this does require some serious bandwidth. On a standard 30mbps fibre connection, it wasn't possible, even though the connection was rated as 'good' (look for a 'great' or 'excellent' rating to avoid issues). This may well be a limitation of my specific home connection - the reason we haven't produced coverage of Microsoft's xCloud yet is that UK ISP Sky seems to have an aversion to streaming platforms. I ended up moving to a Virgin Media connection (rated 'excellent') with a surfeit of bandwidth in order to get the job done. In terms of your connection, Google has a connection checker for determining the type of experience you're likely to get.
The input latency from PC monitors can vary drastically as well.
One monitor released in 2018 was tested to have an input latency of 33.4 ms for native resolutions. While an HP monitor released the same year had an input latency of only 2.6 ms. https://www.rtings.com/monitor/tests/inputs/input-lag
Though I wasn't suggesting that this was the major contributor for the lag, but that there are multiple factors adding up.
What I'm saying about the bandwith is that Google may be sending data of different bandwith volume through different routes.
In this case the data for the PC game may have been forced to travel a longer distance, making more hops before it reaches its destination.
And the paragraph you cited from Eurogamer makes it sound like higher resolutions may only available for PC at the moment.
"We're focusing on the big screen primarily because right now at least, it is the only way to access Stadia's top-end video output - ultra HD at 60 frames per second with HDR support."
But whether that's the case or not, there's also the possibility of being able to chose.
True, further tests would be useful. The game would also render faster if you go for the 1080p or 720p option, thus less latency at the source. If the game can render at 120 fps at 720p, while only sending half the frames, you still have cut the render time in half. In the case of a 30 fps game at 4K, that's 25 ms saved. Also compression and decompression will be faster at lower resolutions and bandwidth, as well as the transmission time per frame. It all adds up.