All corner horn designs like JBL Hartsfield, Vitavox CN-191 and Klipschorn use the corner walls to extend the horn and get lower bass cutoff. Back in the days of mono there was no concern about corner placement affecting imaging and soundstage...it was mono. The speakers size was still some concern but you only had to accommodate one of them. None of these are hybrid or have bass boost (like Avantgarde bass horns, which are part horn and part sealed box). Folded horns are how most people who want true horn bass do it and there are other solutions. The Pnoe is long enough but it is huge. The Aries Cerat Symphonia is long enough as well but doesn't get as deep as a big vented box, for example. Pnoe and Symphonia don't need a corner but the gain from a corner or rear wall will help them as well...when they are well out into a room they can sound somewhat lean.
Corner horns obviously need a corner. But we were not discussing corner horns, and even in vintage horns there are many non-corner horns