My guess is that someone the rosh yeshiva trusts told him the story according to the fanatical kahanist narrative. Rav Mazuz stresses that Goldstein was trying to prevent danger to jews.
The kahanist claim os unfounded. Goldstein was a mentally unstable man who snapped and murdered people; some were probably terrorist supporters, although there’s no evidence of that…there were people who just happened to be at maaras hamachpelah minding their own business, killed for being muslims.
If islam ever had an actual martyr, it would be those people; they were senselessly killed just because of their religion and nationality.