ESD Protection with no grounding
Your internal ESD devices will still take the discharge current themselves instead of letting it through to the sensitive MMIC input node.  This assumes that there is some charge built up between the inner conductor and the shield of the coaxial cable. Some kinds of antennas have a naturally low impedance which pretty much precludes this kind of charge buildup. 

But, as you say, if there is still a charge relative to earth, then it is not discharged until there is a ground path (like the USB plugged into the computer).