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The problem is latency.
CNC needs really precise timing, like realtime kernel precise, which is why the parallel port is on the PCI bus, USB adapters are a recipe for disaster.
I know you googled, but those hits are referring to purpose built USB input motion controllers that deal with timing internally. This thing isn't worth converting to a new motion controller...though somehow it is worth days of my time...
If you can pass through the port you won't have latency.
USB is a shit interface that's barely even reliable for a single 1000hz polling device, let alone two. Yes, you can plug two in and they don't explode, but performance is degraded, drops counts, etc. And each iteration of USB just adds more buffering and latency to try and achieve higher throughput to support things like video output. Mission critical devices that the computer actually requires like mice should have their own interface instead of USB, then just let all the garbage devices occupy the USB slots, and keyboards should have never left PS/2 in the first place! They're still there on laptops.