Misleading to state that this turns your TV or set-up box into a fully hackable device, while the software just runs on a RaspberryPI
Sadly, pmOS doesn't support any TVs as far as their website is concerned (https://wiki.postmarketos.org/wiki/TVs) but with some work (getting the mainline kernel to boot based on the GPL sources or getting the built-in kernel to boot pmOS) you can flash your TV with this firmware. In practice you won't be able to do much with it as TV SoCs are almost always shipped with underperforming hardware and as little RAM and storage as possible, but you could.