Without taking any stance on patent infringement and such, just for information:
- there are different cr3 file formats (meaning, the container format is the same, but the data can be compressed in different ways)
- libraw can decode some of them, and the relevant code was ported (ie. copied, as the license allows it) to RT and ART
- the metadata handling in RT and ART is not done through libraw but through other libraries
- neither the internal rtexif lib nor exiv2 support cr3 so RT lacks metadata support for the format
- exiftool does support the format, and ART takes advantage of it
- RT might also lack some other info (raw crop, black levels…) to properly render cr3 files from the R5: this info is model-specific so it’s not surprising that cr3 works for some cameras and not for others
- ART has such info
TL;DR: ART supports the R5 just fine, as far as I know (except for some compressed options for which there is no open source decoder available yet)