It seems the ones who claim their setup is working are using a 5v -> 12v boost converter cable. If you plug a 5v -> 12v boost converter cable into a port that delivers high-current 5v, then yes it’ll work, but it’s not using the 12v mode of the upstream port, it’s just boosting 5v up to 12v like any boost converter circuit. This requires a high-current capable USB port because- using back of a napkin maths- 5V @ 2A is about 12V @ ~800mA assuming a high efficiency converter.
It’s possible there exist QC2.0 -> 12v barrel jack cables that switch the port into 12v mode, but I can tell with about 98% confidence that nobody in that thread is actually using the 12v power from the QC2.0-capable devices. It needs a handshake to switch into 12v mode, otherwise it would potentially damage any 5v USB devices plugged into it.
In this case, if you use a straight cable that you’ve hacked up to take the USB connection to a barrel jack, you will get 5v @ 2.4A max. A multi-meter check will confirm this.
If you handshake, using the hack detailed above, then you will see that switch to 12v.
If you order a 5v to 12v cable from Ebay, it’ll do internal conversion and you might as well plug it into any sufficiently powerful USB port, since the 12v QC2.0 feature wont be used.