I bought a Poco X3 NFC about a month ago, and also was confronted with the Xiaomi account signup request when I tried to enable USB debugging.
For me this was enough of a reason to send the device back, but I started fiddling around and ended up being able to use USB debugging without an Xiaomi account. I don't remember how I managed to do this, I think I had to disable a specific MIUI optimization. No ADB had to be used for this. I think it was this https://android.stackexchange.com/a/185876
I'm also pretty sure that I did not insert a SIM card at that point, because I was still using the device-to-be-replaced on that and the following days.
I think it's just a lot of tactics which they use in order to push you to create an account, but ultimately it's not required.
That being said, I really despise their MIUI, all their modifications. Everything about it attempts to make you use their products, even if Google's apps are already installed.
For me, the Android experience which the Pixel devices give you are all I want. Even Motorola's minor enhancements are something I don't want on a new phone.