There seem to be jumpers to connect POE into the RaspberryPi. I have a case with two little fans which are meant to connect to the GPIO. I think they could be connected to the pins of the POE but I would like to know if this is likely to cause problems. I don't want to use POE.
As a practical matter, wrt powering your fans you would need to do a fair amount of work to use the power. POE is governed by a rather complex group of standards and specifications. Delivery of power to the RPi's terminals would need to be negotiated with the upstream POE provider. Strictly as a personal viewpoint, requiring a "HAT" to use POE on a Raspberry Pi strikes me as some type of a joke - but personal viewpoints aside, in your case I'd opine there are better and easier (certainly less expensive) methods to power a fan.
If this doesn't fully address your question, please let us know.