This is not even getting to that; but that is actually pretty simple, to be fair.

The negation is: 非常道

非 -- fei, this has been a negator for a very long time in Chinese

常 -- chang, this means something like common, usual, normal, or nominal (I suspect that translating this as "eternal" is a creative move on the part of some westerner)

道 -- dao, the way again

Nowadays, 非常 as a compound is used to mean "extraordinary" ("not normal", "exceptional"); but it is not hard to see how it might be taken to negate 道 here.