The point about the the astro-gazers and winter heat wave in Ghidorah and the magnetic waves coming from Planet X in Monster Zero is an interesting one. I do wonder if Sekizawa had that in mind when he wrote MZ, utilizing that story element from his prior script as a launchpad for why Fuji and Glenn's space voyage begins.
Beyond that, I think what that guy on that site is going for is just a case of drastically overthinking things. There is nothing in Ghidorah to suggest that the titular space demon is being controlled by aliens from Planet X, because that's not what Sekizawa had in mind when he wrote it. The premise of Ghidorah is a simple one: there's a three-headed space monster named King Ghidorah who likes to go around destroying planets; he destroyed all life and civilization on Venus several millennia ago and now he's arrived on Earth. He's an old fashioned monster on the loose with no explanation of origin. That's been the premise of Ghidorah the Three-Headed Monster since the film was made and released in 1964, before the notion of Planet X was even conceived. Ghidorah is the same movie now as it was in 1964, premise and all, regardless of the fact that a direct sequel followed in 1965.
Even then, while Sekizawa could have written Monster Zero to retroactively explain King Ghidorah's actions in the previous film differently, that's not what happened. Nothing in MZ suggests anything to the effect of the Xians controlling Ghidorah when he first arrived on Earth in the previous film. I've heard people point to the line "Ghidorah was always one of their weapons" in the third act, but that line is a product of the English dub. The originally written line (according to Classic Media's DVD subtitles) is "I should have known that King Ghidorah is their tool," and I'd say taking that to mean that the Xians were responsible for the events of the previous film altogether is a bit of a stretch.
It's the same deal with the notion of Princess Salno's Venusian awakening really being the influence of Xian resistance fighters. That's taking Star Wars-type speculation to a level of fan fiction proportions. I could just as easily suggest that the assassins trying to take out Salno were manipulated by the Xians in power to halt the so-called resistance. There's nothing presented in the film to indicate that to be the case, so it's nothing more than fan-dreamt speculation. Merely wanting an idea to be true doesn't make it so.