I haven't been keeping up with the latest USB spec but this direction of power flow sounds like it would have been in breach of earlier USB specs for everything except USB-OTG which turning a device into a host necessitates the reverse of the normal power flow.
I believe USB-C and USB Power Delivery (USB PD) specs had this specifically in mind. They changed the fixed concept of one device being the host/master and one being the guest/slave in lieu of the devices being able to negotiate roles as needed.
In "classic' USB, you had two roles - a host, and a slave (or device). Power flows from the host to the slave, regulated and monitored so the slave does not draw too much power. The host also interrogates the slave to figure out what kind of device it is and all that other good stuff.
With USB-C and USB-PD, the roles are more flexible. First, the direction of power flow is no longer obvious. Something like a monitor may dock a laptop over USB-C, so the monitor is the device (and thunderbolt target too) but power is flowing from the monitor to the laptop to charge it. In classic USB, this can't happen - the laptop is forced to power the screen instead.
In USB-OTG, no power flows at all, though the present host will supply up to 100mA at 5V in case there's any circuits needing powering. If the roles switch, then the old host switches off its power and the new host turns on its power.lh