Conversely, the last time I checked a couple of weeks ago, it was impossible to find any USB4 external SSDs on Amazon; only USB 3.2.
It is probably the speed of it being read into RAM.
Try entering sync right after copying to see how long it really takes
It beats my previous desktop's RAM speed, what a time to live in.
Of course, just give them some time and they'll come up with USB4 "gen classic" at 11 Mbps.
https://www.aliexpress.com/item/1005008555989592.html
I have one of these, though I'm using with a USB 3.x port as that's what my desktop has. For me it's working fine, and for others with actual USB 4 ports it seems to be working properly for them.