A Harvester head in a Youtja trophy room doesn't indicate that the harvesters had been to earth yet. The Harvester could have been hunted on its own home world or anywhere else in the Universe for that matter.

Plus given the circumstances in the Predator 2 film, failure to mention the ID4 events is understandable to me, as the Youtja in Predator 2 provides a scenario that required full attention and strategic action.