I know that the ASM2464 does support USB3 20G connections as well. I have used that also with mine.
But since this thread is about whether the 13th gen USB4 ports also support USB3 20G (which is not mandated by TB4 or anybody else and has so far not been seen in the wild on a USB4 host port), the ASM2464 left alone will always make a USB4 20G connection instead of the USB3 20G connection that I wondered about in my initial post. We already know that all of FWs USB4 ports do support UB4 20G speeds, as that is mandatory.
I have not managed to force my ASM2464 into another mode with the tool (although it runs on all my hosts, including my 12th gen FW), so unless you can force it with software to not make USB4 connections at all, it will not prove anything we do not know about the 13th gen FW or Intel CPUs.
Towards the speeds with USB4 20G vs USB3 20G: I have not looked into the overheads of USB3 and UASP. But PCIe tunneling via USB4 has huge overheads: larger than native PCIe, since TB/USB4 still limits to 128 Byte per packet vs. the 256 Byte that most NVMes want. And USB4+PCIe incurr about 32 Bytes overhead per packet on top of the 128 Byte.
So since there are more layers to USB4 + PCIe + NVMe it makes sense it has less usable bandwidth. But on the other hand, much better latency than translating everything into a dummbed down USB protocol that was not designed for high performance SSDs and will achieve far lower IOPS.
Btw. have you seen any compatibility issues with the ASM2464? My Satechi on release firmware (1005) made only USB2 connections to my TB3 host. With new firmwares (1218) it makes mostly x2 connections (gen 3, I only have a Samsung 970 Evo in it currently).
And then it stopped making USB4 connections to my TB4 desktop (Maple Ridge, TB drivers). The only host it works reliably on is my FW (USB4 drivers of course).
And even here, chained behind a TB4 hub, it limits itself to x2 Gen 3 connections.