This is technically incorrect information and could get people in trouble if followed literally.
On Qubes OS, if a user creates a new identity inside a Whonix workstation disposable VM via the browser's new identity functionality, the new identity spawns within the same disposable VM. I just tested this on Qubes OS 4.3.
That, I assume would expose one to OP's vulnerability, as its still running in the same VM. I would be glad to learn that I'm incorrect in my unverified assumption.
Even Qubes OS users still need to be mindful to launch new disposable VM when keeping identities separate to sidestep this attack.
By you reasoning, Qubes doesn't provide more protection than the underlying operating systems. I've seen this myth on HN multiple times.