Personally, I think he's mistaken or lying. I just wanted to make sure that we considered the reasonable alternatives.
I'm pretty sure in reading TFA there is little chance he could be "mistaken":
The secret sauce seems to be ultra-dense deuterium, "D(0)" whatever that means. Looking through the author's other papers, it looks like he's claiming to have made metallic hydrogen, which would be a Nobel Prize right there. And it's starting to look a little bit suspicious that no other labs have replicated the work in the intervening eight or ten years.
While metallic hydrogen probably exists inside the core of Jupiter, no lab on Earth has succeeded in making metallic hydrogen repeatably, although it's been postulated to be possible since 1935 and many have tried. Teams at Cornell and the French Atomic Commission have both given it a shot, and failed with pressures as high as 3.2 million atmospheres.
Well, no labs except [Holmlid]'s. It must be true, though, because it's on Wikipedia! It says right there that the [Holmlid] lab made metallic hydrogen using "Rydberg Matter". We'd never heard of this stuff, so we followed that Wikipedia link down the rabbit hole, only to find some mumbo-jumbo that we didn't understand and citations of papers nearly exclusively by, you guessed it, [Leif Holmlid].
If he can demonstrate this, then fine
But I'm sticking with my "if he can't demonstrate that it works in such a way as to be repeatable by someone else, then he must be a lying, attention-seeking media whore."
It isn't up to the world to validate his outrageous claims. Put up or shut up.