Well, the Warhammer, Marauder, Archer, and P-Hawk were all released a while after HBS initially announced they had made a deal and were using PGI's game models. It is eintrely possible these newer models were not part of the deal. I mean, if I was PGI, I wouldn't make a deal for all the IS mechs I've got plus any others I might make in the future for set terms. I'd make a deal for what I had, and then if they wanted new stuff later, that would be a separate deal.
As further evidence of this, the Assassin and the Javelin are both NOT unseen, and I don't believe either currently appears in the HBS mech list, even though they are correct for the time period and PGI has models for them.