Making a net energy producing fusion reactor container sized is a feat, making your first prototype container sized is just starting out. The first tokamak had a radius of 0.7m, so could easily fit inside a standard shipping container.

I don't think they have the ability to keep a secret like that. A couple reasons: 1. The act of keeping the secret makes the tech useless. If it's not incorporated into equipment that could use it (e.g. a naval vessel, an isolated firebase) then what good is it? And if it is incorporated and there's a bunch of 20 year old techs working on it, it's going to leak. 2. I don't think they're that bad of people. If they could save us from global warming with fusion tech, not doing so would be a great tragedy. And historically at least the US military just isn't that selfish. For example, they opened up GPS even when they didn't have to. I think they get that a richer/stronger public means a richer/stronger military.

No, they wouldn't be issuing press releases if this was a black program. You're also not just taking Lockhead's claims at face value, which themselves have not be established with evidence, and going further to imagine that they're lying/slow playing their claims and actually are even further ahead. None of this is motivated by evidence.

I don't know how a basic understanding of how military technology is developed fits into "conspiracy theory".

I'm not projecting that a sinister cabal of technologists are hiding some great truth, just that the department of defense and defense contractors are obviously developing this technology and that new military tech is usually highly classified. There's no great mystery there.