To further clarify : On Arch Linux, the new kernel and its corresponding modules are installed on the system, but they are not immediately loaded into RAM. Until you reboot your system, the old kernel and its associated modules remain in memory. Once you restart your system, the new kernel is loaded into RAM, and the old kernel and its modules are unloaded from memory. From that point onward, the system uses the new kernel and its modules.

If the modules from the old Kernel weren’t loaded previously then they won’t be loadable into RAM when you say pluggin a USB device, but USB devices that were already plugged in will continue to work because the module was already loaded into RAM from the old kernel.
You’d have to unplug the USB device and wait for the system to unload the module from memory automatically(which doesn’t happen often because it’s in best interest for the system to keep the module around in case it’s needed again) Or unload it yourself for problems to occurred.
By unplugging and replugging the same USB device during this time, it’ll still work as the model wasn’t immediately unloaded.

The package you shared makes a copy of the old kernel modules on disc to a different location prior to the Kernel update process replacement of the old Kernel as to load them if they’re not already available in RAM.