"Aimless drift" of DNA is also evolution.
And contrary to your claim, if we were convinced that the spider died 20Mya, and that DNA was not contaminated etc., and the DNA were the same as some modern species, some fundamental assumption of the theory of evolution would be proven wrong.
1. "Aimless drift" of DNA is also evolution.
No, it is not. There is a lot of DNA that (appears to) to do nothing. Note, this IS a related concept, in that you can track the evolution of a species by looking at similarities in the junk DNA --- the idea being that there is no pressure to change the junk, so if the same exact sequences of junk (which serves no purpose and appears to be random, remember) appears in two species, then that is some evidence that the two species likely had a common ancestor species (or one species was the ancestor of the other.)
For example, certain primates and humans share similar unique "junk" DNA, which SOME opine is SOME evidence of common ancestor.
2. " . . . and the DNA were the same as some modern species, some fundamental assumption of the theory of evolution would be proven wrong."
No, it would not. Time does not necessarily equate to evoltion (although it often does corrolate). If there is no predator, competitor, disease, envirnomental change, happenstance isolation, or whatnot to force a change, there will be no change --- there is no "pressure" to change.
That is the fundamental basis of the evolutionary theory --- no pressure, no change. Pressure, change.
That said, this is an entirely new species of nasty spider, and, just looking at morphology (shape) the DNA will be different than the species of spider that are alive today, so the dispute is moot.