It’s another case of OSPNS - Open source projects names sucking .

You can register on ibis.wiki, open.ibis.wiki or other instances.

Correction, they are not listed as open to registration on fediverse.observer and they don’t have a register button on the front page. unlike fedipedia which is opened.

Also some instances appear to be missing from fediverse observer (i see at least opentextbooksolutions.org not appearing). you could feed it manually which should work (i did it for opentextbooksolutions.org) but a automatic addition will be better. also a lot of the other instances listed on the page you linked appear dead.