In full, the paragraph reads:
NOTE that the Raspberry Pi 3B+ and Pi Zero and ZeroW do not include an input ZVD. The micro-USB input on a Pi is expected to / almost universally is driven by a power source which does not sink current, i.e. it will not try to actively pull down a voltage higher than its regulated voltage. If a HAT back-powers a Pi and uses a power source that does not try to sink current (and will safely stop/pause regulation if its input voltage is higher than its regulation voltage) it is OK to not include a ZVD on a HAT. If you are unsure or don't know then please include the ZVD! The HAT designer is responsible for the safety of their product.
So this question isn't really about the Pi itself - rather it's about whatever power supply is connected to the Pi.
"is expected to" means they designed the Pi that way - no further explanation required.
"almost universally is" can only mean that (a) the official PSUs, including the PoE HAT, comply with this, and (b) they haven't yet come across a readily-available supply that doesn't comply.
It doesn't mean I couldn't
design a supply that does
sink current - a shunt regulator using a 1N5338BG would do this quite happily.
It only means nobody in their right mind would do it.