You're correct, that previously, platinums have only been produced by breeding two normal-looking dempseys that carry the genes for both gold and blue. So only 1/16, or about 6%, of the fry were platinums.
BUT as far as I know, this is the first time an actual platinum has been successfully bred. Since he and his mate are both gold, all the babies are gold. Since he is also electric blue, and his mate carries the gene for blue, half the babies should be electric blue golds...AKA platinums
If the platinum male had been bred to a female with any other combination of genetics, that would change everything, and there would be a far less number of platinum offspring.
Later today I will sit down and draw up a Punnett square and post it...it would probably make a lot more sense that way...
BUT as far as I know, this is the first time an actual platinum has been successfully bred. Since he and his mate are both gold, all the babies are gold. Since he is also electric blue, and his mate carries the gene for blue, half the babies should be electric blue golds...AKA platinums

If the platinum male had been bred to a female with any other combination of genetics, that would change everything, and there would be a far less number of platinum offspring.
Later today I will sit down and draw up a Punnett square and post it...it would probably make a lot more sense that way...