is "MUST only if" a MUST?

Hi there,

	I cannot decide if the following is a MUST-level requirement
(i.e., its violation prevents RFC 2616 compliance, even conditional):

   13.10 Invalidation After Updates or Deletions
   ...
   In order to prevent denial of service attacks, an invalidation based
   on the URI in a Location or Content-Location header MUST only be
   performed if the host part is the same as in the Request-URI.

Suppose the host part is not the same as in the Request-URI. Let's
also assume that the device did perform an invalidation, subjecting
itself to a potential DoS attack. Did the device violate a MUST-level
requirement? The answer seems to depend on how you bind "only":

	[ ] Yes, this is a MUST-level violation because
		foo MUST only blah if bar
	    implies
		if not bar, foo MUST NOT blah

	[ ] No, this is not a MUST-level violation because
		foo MUST only blah if bar
	    implies just that
		if bar, foo MUST blah
	    and requires nothing when bar is false ("if not bar")
	    
I suspect that the intended interpretation is "yes, this is a MUST
violation". Can anybody confirm? Is there really a problem with the
wording, or am I imagining an ambiguity?

Thank you,

Alex.