Aspects of the Yasen design may date back to the 1980s, but it's still of far newer vintage in every way than the Victor III, the last unit of which was launched 2 years before Severodvinsk was even laid down. Most silencing technology is impossible to retro-fit, including the reactor (where the first Yasen unit is 1.5, the second 2 generations ahead of the Victor III) and raft-mounting arrangements (on which a surprising amount of info has been leaked for Yasen - it is extremely advanced in this regard).
Quite apart from that, I didn't find where he states that in the first place - I'm also not sure how much faith you can put in his info in general. Giving him the benefit of the doubt, he at the very least is dumbing things down quite a bit for the general audience on those topics where I'm reasonably fit to judge. It may be "maskirovka" to avoid coming into conflict with any confidentiality agreements he is bound by, or he may not be quite what he claims, but in any case, bring along salt.