Presumably he meant: why is it deemed forewarned toward gentiles and not deemed forewarned toward Jews (according to the one who says that).

The topic of "forewarned" status is an example of a presumption established by three occurrences. The Talmud treats this as uncertain induction, and therefore any slight refutation is enough to prevent a presumption from being formed (see Yevamot 31, where the contingencies are much more remote). The assumption is that the burden of proof that the ox is forewarned rests on the injured party, even though regarding the basic laws of tort damages the Pnei Yehoshua and the Chazon Ish disagree whether the burden of proof is on the damager or the injured party, because here they are removing the ox from its established status and not merely extracting money from the current holder.

Anyone who hasn't seen dogs bark specifically at Haredim and people dressed in black asked this question. Those dogs clearly have something against them. It's possible that the gentiles of old were like today's Haredim.