Does the secret sauce in NXT prevent a 51% attack by a stakeholder who had a majority of the stake but no longer has it? Maybe but saying "somehow" isn't sufficient. No explanation of what is publicly available provides a credible reason why the attack does not work. Claiming it is solved based on what is not publicly available is well not very useful. If it really does solve it then eventually it will be publicly available and clear to everyone right?
A key point here isn't that I believe it's impossible it's that I believe its impossible absent some trade-off some other attack the system becomes vulnerable to, for example or consideration/cost, or assumption (e.g. that no past majority of shareholders will ever act against the interest of the system, even if they've long since left it), so what exactly is that tradeoff? Keeping the solution secret also implies keeping the vulnerabilities it creates secret, which means that people will be all the more exposed to them.