[exim] delay not kicking in

In response to the recent RCPT-flooding attacks, I changed my

acl_check_rcpt verification check to say:
   deny 
    domains = +local_domains 
    !local_parts = postmaster 
    !verify = recipient 
    message = Unknown user 
    delay = 5s 

When I test non-existent addresses with a dummy smtp session using
exim -bh
then I see the delay being applied:
>>> check delay = 5s
>>> delay modifier requests 5-second delay
>>> delay skipped in -bh checking mode


However, in the exim log file I'm still seeing 99 denied RCPT commands
all with the same timestamp.

How can this be?

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