Propaganda only works if there’s truth hidden somewhere. It’s unlikely it was completely made up. It’s also not worth putting on a temple unless it’s somewhat agreed that these people were a serious threat.

Imagine Trajan making a column and it’s just 90% bs. Would not fly.

The only hard evidence we have of the sea peoples is from Egyptian sources. It depicts a barbarian sea faring group. It’s extremely unlikely (read: impossible) that they were from any kind of land locked area. There’s also no evidence that Northern Europe or Central Europe had developed sailing to the level of sophistication needed.

Most historians see the sea peoples as a coalition of raiders strongly backed by Greeks and Mediterranean people.