I've lived in both halves along the bottom and it's quite accurate. That said, a lot of people in Camarthenshire & Pembrokeshire seem to have quite southernly English accents but are Welsh, a lot are native Welsh speakers so that might give the impression that there's a lot of English people there.
Driving through FUCKING NOTHING is amazing though, stunning views and countryside. I've no idea how the random pockets of people can live there though and not go insane.