He doesn't have to have seen it. But in this case, there isn't much of a case without it. The FSTs (which everybody thinks they pass) is often used to provide this evidence (it also, in some states, provide the probable cause for the arrest and compelled chemical test).
Which is why nobody in their right mind would submit to an FST or a roadside breathalyzer (though a lot of folks are utterly informed regarding their rights and obligations when interacting with policy). If the officer has probable cause, he/she will simply detain/arrest and take to the station for a proper breathalyzer or blood test (which is what's actually required by the implied consent laws; most (or all) don't apply to PBTs, but only the "official" ones at the station. Officers request FSTs and PBTs to develop evidence; you don't have to help them do that.