D-port and round port heads are quite different.
The difference is easy to see, especially from the exhaust port flange which is where the description comes from:
http://forums.maxperformanceinc.com/...d.php?t=636309
Just because the exhaust port floor is flat on aftermarket round port heads doesn’t mean they’re a D-port head.
Pontiac D-port heads almost should be called oval-ports because the exhaust port shape is oval, the two center ports squeezed in back to back are the only D-shaped exhaust ports.
That link to the Butler website only half explains the difference between the two exhaust port types. Either one could be D-port from that picture they show and it just raises questions rather than give any clear answers.
For the uninitiated among us they should have included a picture of at least a factory round-port head that actually does have perfectly round exhaust ports. Also a factory D-port is a lot more D-shaped than an aftermarket aluminum D-port exhaust.
If you’re using headers running round-port heads is head and shoulders above D-ports. Getting the bolts in and having access with a wrench on round-ports is so much better that once you’ve had round-ports you won’t ever want D-ports.