Not true. It will only route the USB2 ports on XHCI to EHCI. USB3 ports continue to work on XHCI.
Not on all Macs. Some (mostly older) Macs still use EHCI. They do the port forwarding for USB2 on XHCI dynamically though and in a way not completely understood. Since FakePCIID_XHCIMux works for me, I haven't investigated further.
They work around the issue by having:
1) accurate ACPI implementation for _UPC
2) port injectors where that is not possible (eg. they didn't want to revise the Mac firmware)