I know I'm not the target market for their UPS addon, but is 91.00€ for a UPS board [1] that does not include the required lead-acid battery sound reasonable? I know they have gone through the certification process for everything, but doesn't this seem fairly high?

The battery is not lithium, but lead-acid. For an application using a Raspberry Pi, I generally assume size is a fairly major consideration. Considering a lead acid battery will need to be three times the size for similar charge capacity, doesn't this battery chemistry choice seem very strange?

A quick google brought me to the PiJuice HAT [2], a $54 piHAT, with an included 1820mAH LiPo. This device also doesn't massively increase the size of the Pi. PiJuice claims "~4 to 6 hours in constant use". Let us assume this is a vastly inflated figure designed by the marketing department. Isn't 2 hours run time more than enough for a UPS? If the power failure is longer, I want my UPS to allow me to safely shut down my gear.

1: https://www.sferalabs.cc/product/strato-pi-ups-board/ 2: https://ameridroid.com/products/pijuice-hat-1