Anythings possible, sure, but that clearly is conspiracy theory category.
Unless they found a working cold fusion method by chance, it would require a HUGE budget and the best of the best scientists avaiable. Who all need to keep their mouth shut and pretend fusion is still not working. For years. Not likely. Not in peace times.
There have been plenty of very expensive defense programs that were kept quite secret for a very long time.
I’ve known and worked with people with top secret clearances, they just didn’t talk about their work, it’s not that hard.
And until a working fusion reactor is build, it is indeed a fundamental physics problem, with all the energy involved, which is not at all completely understood since we are in quantum mechanic level down there.
If just one of them had a startup bone in his body, we would see a small fusion reactor startup making consistent progress that seems amazing. It's not happening.
It's a situation that is apparently very difficult to understand for many people. You work on a classified program and you understand the situation. Nobody goes off and decides to turn their classified knowledge into a business.
I get the value of the myth, people instinctively acheive what they believe is doable. It's just a myth though.