פירוש על עבודה זרה 148:2

Gemara. But if he trampled [grapes] in it [i.e. the vat]: without tarring, scouring is not necessary, rather rinsing [is sufficient].

He taught the customary case: because it is the custom of the tar-sealers to throw wine into it [i.e. the vat].

Rava emphasizes that scouring is necessary only when the non-Jew covered the winepress with pitch. If he tread grapes without lining the winepress with pitch, the winepress does not need to be scoured. We might have thought that it is always necessary, therefore Rava had to recite his teaching.