Where does it say anything about the tank size? Maybe it was a typo and they meant 5-10 gallons. The fish are less than 3 inches long so I can't imagine they'd need that much space.*
One of these days I will get a wild type Betta, in spite of the price, and I'd also love to have a Plakat (5 star fighter) as well as the usual pet variety Bettas. That won't be for a few years though as 6 fish is the maximum I can handle now.
This company's website has an amazing gallery of some of their show Bettas and their breeding facility. They have several double/double ray show Bettas. Two of their betta fry were born as conjoined twins joined at the belly, poor things. They have pics of that. I didn't know that happened to fish too.
http://www.fantasy-aquatic.com/gallery/index.php