Yes, you are right. RiscOS is continually evolving and there are now IMAGES that can be burnt direct to an SD card (Etcher) for all versions of RPi, but these image files are not installable by NOOBS/PINN for a multi-boot scenario as the OP requested.
RiscOS has some special file system which required additional support to be built-in to NOOBS, and a special BLOB file to be created. I understand that a new BLOB may need to be created by the RiscOS maintainers, and new NOOBS format .tar.xz files for the latest OS version.
The version that is installable by NOOBS is now quite old, so it does not support the latest RPi models.
I would be happy to add an updated version of RiscOS to the list of PINN installable OSes, but I have no idea how to create the necessary Blob files. So the RiscOS maintainers will need to be asked to create the NOOBS installable files. It may need modification to NOOBS/PINN too, depending on the requirements of the BLOB file.
EDIT: See viewtopic.php?t=193763#p1214114
which is why I didn't go any further last time.