It isn't just this breeder. I don't believe it is a "line" thing as all DT are all prone to SBD. Wish indjo would come on. He explained it to me; don't want to appear it's my knowledge.
DT is a recognized fin type by the IBC and is described as "Doubletail Bettas may have a deeper--top to bottom-- body than singletailed Bettas. The thicker body is acceptable, as long as it contributes to the support of the larger finnage of doubledtails." (Please note, I did not fix typos.)
Doubletail Bettas are expected to differ in several ways from the singletail
1. Possess two distinct “tails” or caudal lobes instead of one, with a complete separation to the base of the caudal peduncle.
2. Possess a wider caudal peduncle to support the double lobes.
3. Possess a larger dorsal fin, nearly the size of the anal fin.
4. Their bodies are usually more “chunky” and often a bit shorter.
5. Bends in the caudal peduncle are expressed to a varying degree in almost all doubletail bettas. These are more easily noticed when viewing the fish from above. Easily seen bends should be faulted; if the bend is not excessive when viewed from above, the fish should not be penalized."
I like DT and have had them. But, at least to me, they look more like Goldfish in body. And we know how prone to bloat those are.
Anyhow, TMI. Sorry.