There's a reason Wikipedia isn't an acceptable source in college-level courses.
In any case, a permalink to a specific revision that includes a sha256sum of the article is a good way to ensure you're getting a reliable link to information which can not be tampered without failing the checksum.
Sometimes, Wikipedia editors don't understand the source so the Wikipedia article and the source are actually at odds with one another. I've witnessed this a few times when I investigated dubious claims.
Adding that citation is one of my proudest Wikipedia edits ever.
If a person comments on Reddit with a valid point and sources all his facts, why does it matter where said comment is made, and honestly, it doesn't matter if admins can edit it or not, really.
Look at it like posting an EXE and also linking to a credible site containing it's checksum. As long as you trust the linked site, then it doesn't matter where the EXE is posted and if someone has access to modifiying the EXE, all that matters is that 1. you got the data, 2. the data checksum matches the source.
Congress was looking at evidence of what that particular user was doing online at the time, because /u/stonetear posted questions that look rather incriminating in retrospect based on what we've learned since then.
This kind of thing is why it's important to establish a chain of custody for evidence.
If the reddit admin edits of comments aren't appropriately stored in comment history, the logs turned over won't tell the whole story, but reddit will (mistakenly) testify that it's the complete history.
You can even add a dash of malice: an exec edits a rival's post, but the subpoena is filled by a line tech (possibly unaware of the admin tools, even).
Unless the defense knows to press reddit on the actual veracity of their logs and ways they could be compromised, the erroneous data seems a fact to the court.
But Wikipedia IS good for finding those primary/secondary sources. And, depending on the topic, wiki is great for getting a quick primer to a subject or to have an example and is often cited in lectures to students who are expected to be old enough to not stop you and say "Teacher! Teacher! You said to never use Wikipedia!"
There's a lot we don't know without knowing the internal workings of Reddit, though I agree that you can't have to think that if one rogue person (even someone like that) can just do such things, they don't have robust internal controls at all.