Those of us who have lived with the piano all our lives are more than a bit spoiled by it. Just as anyone under 20 has a hard time imagining life without computers anyone born after, say, 1800, would find it difficult to imagine life without pianos.
It is perhaps impossible to imagine just what the impact must have been of all that resonance all of a sudden emanating from a keyboard instrument. People who describe the sound of the forte piano as thin are comparing it with the modern piano. The closest anyone might have come who was alive in 1727 would have been the sound of the baryton or of the viola d'amore--stringed instruments with unfingered strings in constant sympathetic vibration.
Haydn was certainly writing for an instrument he heard as fully fleshed out because, to the ear of the 18th century, the forte piano was richly resonant.