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.
Well, first of all, if the defense are suspicious, they could subpoena the Way Back Machine for their logs regarding that particular archive website, e.g. file modification dates, backups, logins to the system, and so on to see if after the initial 'scrape' of the site was changed later in the saved version of the site they have.
The defense can also introduce their own rebuttal evidence, e.g. the accused still has their own copy of the site from the same time, and it is different from the Way Back Machines.
And like every other piece of evidence, there needs to be someone to 'vouch' for it. This can be in the form of eyewitnesses or expert witnesses.
e.g. CCTV footage of a crime can't be admitted unless the admitting party can prove its provenance, i.e. "My name is Joe Blogs, and this is the video tape I took from my shops security system minutes after the incident. The timestamps displayed are accurate, it is a security system with only one camera attached to it, and it was functioning normally at the time of the incident, and between removing it from the recording machine and handing it to the police, no-one else but me has touched it and it has not been tampered with."
Substitute the Way Back Machine for CCTV system, and Joe Blogs the shop owner with various system admins/management at the Way Back Machine.
That is how all evidence works. Someone presents evidence, attests to it, and then it is up to the other side - or the judge - to present reasons why it can't be accepted. That is the whole point of rebuttal testimony, cross-examination, character examinations and so on. When a defense attorney asks Joe Blogs from my example above seemingly irrelevant questions like whether he is having an affair, or is a closet homosexual, or has a police record, or about his taxes, he is trying to cast doubt in the minds of the judge and/or jury about how reliable this person is, and maybe you shouldn't believe anything he says about the submitted CCTV evidence because he is a known liar, etc.
Exactly the same process will apply to the Way Back Machine, the opposing counsel has the opportunity to examine the providence of the evidence, and can bring in their own expert witnesses to cast doubt on the evidence, to show, reasonably, that it could have been tampered with.