Basic Principle 1: a civilization VIRTUALLY NEVER LOOSES ITS TECHNOLOGY. Therefore, unless tech loss is canonically stated, all what civilization knew before it knew forever. The only exception is when a culture is (nearly) wiped out
Basic Princible 2: If many civilizations merge into one, the new one has the combined knowledge of both, including all tech ny of them possessed.
We know from "Enterprize" that after the vulcan contact there wasn't a big war on Earth till the beginning of the show and by then, all terran civilizations merged together, including EH.
So whatever EH knew, was merged into Earth Government culture, and since Earth was never wiped out (in main timeline), it retained al EH knowledge forever.