It seems likely that, if we go by Kiriha's surprise at learning Usato came from another world, average citizens (at least) don't believe the first hero was isekai'd, and think that's a fairytale. Plus, Usato and the others weren't summoned by an alliance, but by a single kingdom. So it might actually be possible that only Lyngle kingdom has the knowledge to summon heroes, or that no other nation knows the first hero was isekai'd.