It could be that % was a typo; it's Shift-5 for % vs Shift-4 for $ on the keyboard, and the corresponding UNIX opt line doesn't have a % in.
I don't know whether removing the % will make any difference, but I might try building the Amiga executable in NetBSD without it.
The
BCPL manual has a section on conditional compilation on page 13 (p.21 of the PDF).