(Disclaimer: no experience myself, just remembered seeing multiple HATs in this area in the past so did a quick search)
"PiJuice will use up to just five of your GPIO pins (just power and I2C), the rest are free to diversify your project. The stacking header allows you to continue to use your existing HATs and add-ons with PiJuice." from https://uk.pi-supply.com/products/pijuice-standard
But sure, some people/projects might prefer to shift the hardware to not be attached to the Pi. I wouldn't be surprised if you could find a product that does that too, or at worst I presume a basic plastic box (there's probably pre-made ones for the size of various Pi HATs? and of course 3D printing) and some fairly simple DYI wiring to connect the 5 pins through a cable wouldn't be too tricky.
The hat is a great idea in principle, but because the battery is sitting right on top of the processor, it doesn't stand a chance given that lithium batteries are fairly temperature sensitive.
I'd go with another option.
Hit Amazon.com and search for "raspberry pi UPS" and you'll find plenty of options. Or more generically try search terms like "mini ups" or "router ups" or "dc ups" - loads of options at places like Aliexpress as well.
Here in South Africa with rolling blackouts happening fairly regularly these days a lot of homes have something similar to keep internet connectivity live when it happens, with a lot of those options having USB outputs as well.
I'm currently using one of these to keep my pi 4 + hdd setup online: https://acconet.net/product/ups-rp7500a/
https://aliexpress.com/item/1005003164049579.html, https://aliexpress.com/item/1005004107179526.html (POE)
https://aliexpress.com/item/32964061367.html (wall wart style)
https://aliexpress.com/item/1005003765804094.html (looks like a power bank with always-on output like the article is describing)
https://aliexpress.com/item/4001204992959.html (BYO 18650s and enclosure)
https://aliexpress.com/item/32421259702.html (looks interesting but sadly "dead", was cached in google search results from however long ago)
If it's just PoE you're looking for there are other options available - e.g. https://www.amazon.com/Battery-Uninterruptible-Security-1000...
One thing to be wary about though is max power if you're trying to run a raspberry pi with hard drive attached off one of these devices. A lot of these devices cap output on the USB ports at ~5V/2A (which I think was the USB2 max whereas USB3 allows for more). If you're running off an SD-card or SSD you should be fine, but 2A isn't enough to start a pi from an attached 2.5" HDD in my experience.
Ironically I got mine before I learned of the power cuts in my new home while building it. I was just merely thinking of edge cases of how a smart thief could undo my security cameras and figured cutting off the power would be one option I need to prepare, so got this UPS to backup both the modems (and redundant internet connections) and the home bridge for the battery backup security cam.
Still a nice option!