as far as I know you are correct. the original platinum, when bred to a normal female, produced only lessers and normal-looking offspring that were 'het for platinum.' in order to produce a true platinum, you'd have to pair a typical lesser with a 'het daddy' to use RDR's terminology.
I asked a similar question not long ago about whether the platinum 'hidden gene' is allelic with the entire leucistic complex and it seems to be. I know I saw a clutch of 'mojave daddy' offspring on bush league from a mojave x het platinum pairing.
I think using the term 'hidden gene' is a misnomer but the normal looking offspring from a platinum do indeed produce a different snake when paired with a lesser. I also don't know why there isn't more work being done with the platinums themselves. perhaps mr. davis keeps those stock back more than not or perhaps there just isn't sufficient interest when lessers already look as great as they do.