From what i know anything marked “Free” on epic store is free, as long as you use assets that are free for unreal in unreal and free for Fortnitein Fortnite, nobody will complain.
Even some Unreal assets are allowed to be used in Fortnite, not sure other way around.
So those Fortnite free assets are safe to be used in Fortnite.
However unsure about them in unreal projcet.
ps.
Since they are not characters or weapons i think nobody would protest if you used them in unreal project.
Thanks for the answer bro