Perhaps the technology was agreed to be forbidden by the Thunderans generations ago, illegal, taboo. Over time, the exact origins of that taboo were forgotten, and it became more mythical than anything else. Fakes became common... like superstitious talismans and whatnot today, perhaps. Fools BELIEVE it may be real, but it's all charlatans making a quick buck.
It's completely plausible for the Thunderan Empire--which is merely the most powerful in the world, not the RULERS of the world--to not know every corner of Third Earth. Could be they pulled back from areas where they hid tech in ages past, and over time forgot a lot of it. History becomes legend, legend becomes myth, etc... facts become warped, or forgotten outright.
And Mumm-Ra joining the lizards and Grune, that there is enough explanation for now. We'll undoubtedly find out some more as the series continues and the T-cats get closer to the Book of Omens (and find Panthro and whatever), but the lizards... they have their own lands, not as fertile as the Thunderans. They're repressed, but not a conquered slave-race. It's no stretch to think that Mumm-Ra saw them as tools, made them a deal, and revealed the whereabouts of ancient tech--or plans, in order to make NEW tech. Or he had it all in his pyramid. If he even has a pyramid. We don't know. Whatever way, it'd work. The lizards wouldn't question it too much, even if they had similar beliefs as the Thunderans about high technology. "So, this crazy stuff unlike anything we've ever seen, stuff we consider strange new sorcery... this stuff will let us have revenge on the Cats for their treatment of us for so many generations? And all you want is their Lord's fancy sword? You've got yourself a deal."