The SRFI-18 spec says:

Primitives and exceptions

When one of the primitives defined in this SRFI raises an exception defined in this SRFI, the exception handler is called with the same continuation as the primitive (i.e. it is a tail call to the exception handler). This requirement avoids having to use call-with-current-continuation to get the same effect in some situations.

However although we do return to the continuation of the primitive, the handler is first invoked from the continuation of the error, not that of the primitive. See the `wrap' procedure around line 250.