I think I agree it was a mistake/wart. I can understand the frustration of the GMP dev in the text - they get hit hard by this. The omission feels arbitary and capricious and a bit ideologically motivated. The defense of the choice seems like post-hoc rationalization.

RiscV looks like a nice ISA otherwise.

I wouldn't be surprised if it was eventually extended to add a set of variant instructions that wrote flags to an explicitly specified register.