I have to blame myself. I just checked the history of Chibi. Four years ago, I asked Alex to change `raise-continuable' into `raise' because there was an erratum #17 of the R7RS. But that erratum had to be errated itself (by removing it), but Chibi still has the version according to erratum #17.
(Alex, I included you in this post.)
The confusion came from an erratum from R6RS in §7.1 ().
So we see that `guard' does exactly the right thing. It would be a bad sign if it couldn't be used to implement `either-guard' because in that case it would do the wrong thing with respect to continuable exceptions.