Is this safe?
Looks like probably yes. I also found the file listed on https://packages.ubuntu.com/focal/armhf/ciso/download. I don't see where to actually download it from this page, but the md5sum given there does match the file downloaded from the other source. This looks like a rather official source, even if I don't know what Launchpad Librarian (the other one) is.
Will it work?
Also, probably yes. It does run. I compressed two ISOs with ciso on my RPi, and also with umdgen on my PC, and compared them.
For one game, the CSO made by ciso and the one made by umdgen, were identical (yay!), and on decompressing, the output ISO was also identical to the original.
For the other game, the CSOs were different, but they both worked.
On decompressing, they came out still different, but they also both still worked. The one that was made by ciso, when decompressed by ciso, was identical to the original source ISO (yay!). But the one that was initially made by umdgen, when decompressed by ciso was different than the original, but it did still work.
I got about 20% savings overall (10% on the big one and 45% on the smaller one.)