I agree but at least with normal evidence there are at least some safe guards, what do you have with this? It's not like you can cross check against something else.
You seem to be ignoring the legal arguments entirely: evidence is not proof. Evidence can and, unfortunately, frequently is tampered with which is why corroboration of different kinds is required. What the decision does is set a precedent that the archives of the Wayback When Machine can be admitted as evidence. This is no different to when fingerprints, and later, DNA or other forensic items were considered admissible: both of which can, and infamously, have been manipulated.
In any particular trial, however, a court can decide whether any particular piece of evidence can be considered admissible, which provides ample scope for defence counsels to challenge.