That is not true in the slightest because a St. Bernards heart and lungs are proportionate to its body mass, a Bully whippits, heart and lungs are proportionate to a normal sized whippit, its not hard to understand how this could be a problem, Im not an expert but after reading this its pretty easy to understand.
I've trained 2 bully whippets personally and my aunt has one. These dogs are bred accidentally -in every case I've been involved in at least- and need owners who love them. They can live long, healthy lives (my aunt's is currently 9 and in great shape, only minor signs of hip displaysia) and as long as you don't strain their hearts or muscles more than they can handle, they can be treated like normal dog breeds.
To say that the breed is in severe pain and always die young or constantly suffer from muscle cramps is simply not accurate. They have issues like all breeds.