The output is absolutely correct.  Note that in your case

.bb (&x ^= POSTMASTER) and (&x ^= OWNER)

evaluates to TRUE, so the conditional block is executed.  You didn't say
which block was executed after that but I'm assuming it's the "other people"
block.  And that's correct--it <is> possible for "other people" (in this
case only the list owner(s) and postmaster(s)) to see that someone is
subscribed to the list.

Naturally you can feel free to discard this block and hard-code the message
if you want.  Note that you left out the closing .eb that should follow
the line

just issue a "SET &LISTNAME CONCEAL" command.

however.

Nathan