The officer had not heard to bicyclist's side of story at that time and he was likely saying that to appease you. Lucky for you because only based on your description (assuming this echoed your statement to the police) you could have been cited for failing to yield the right of way.
There is no law in Pennsylvania that shifts liability to a speeding driver. So even if he was "
going pretty fast", (1) it would be best to stay out of his way, (2) "pretty fast" is fairly subjective, (3) you obviously didn't see him approaching and therefore you don't know how fast he was going and (4) even if you did see him, you are not qualified to provide a speed estimate that is reasonably or remotely reliable or accurate.
There is no one doubting that you stopped. Stopping, however, is meaningless if you are not going to yield the right-of-way to approaching vehicles that are not required to stop!
§ 3323. Stop signs and yield signs.
(a) Intersections controlled by signs.--Preferential right-of-way at an intersection may be indicated by stop signs or yield signs as authorized in section 6124 (relating to erection of traffic-control devices at intersections).
(b) Duties at stop signs.--Except when directed to proceed by a police officer or appropriately attired persons authorized to direct, control or regulate traffic, every driver of a vehicle approaching a stop sign shall stop at a clearly marked stop line or, if no stop line is present, before entering a crosswalk on the near side of the intersection or, if no crosswalk is present, then at the point nearest the intersecting roadway where the driver has a clear view of approaching traffic on the intersecting roadway before entering. If, after stopping at a crosswalk or clearly marked stop line, a driver does not have a clear view of approaching traffic, the driver shall after yielding the right-of-way to any pedestrian in the crosswalk slowly pull forward from the stopped position to a point where the driver has a clear view of approaching traffic. The driver shall yield the right-of-way to any vehicle in the intersection or approaching on another roadway so closely as to constitute a hazard during the time when the driver is moving across or within the intersection or junction of roadways and enter the intersection when it is safe to do so.