One of the fundamental rules of evidence in a legal proceeding is that a statement made by an opposing party that is against his own interest is pretty much the gold standard for evidentiary value. If you make a statement in or out of a courtroom that supports your position, that’s expected. That’s not actually helpful to your case except inasmuch as it is an absence of statements that undercut your position. Your right to remain silent in a criminal proceeding is not to protect you against your protestations of innocence but rather the attempts by investigators and interrogators to get you to say something that will undercut the final version of your story. On the other hand, if it is established with some degree of certainty (such as in writing) that you have stated something that is harmful to your interests, the logical process works like this: you have a reason to lie to make yourself look good or help your own case, but if what you say actually hurts your cause, there’s no reason why you would tell a lie that undermines your own interests. Therefore, it is much more likely to be the truth.
Furthermore, you can be a real lying bastard 99% of the time, but that one time when your statement gives ammunition to your opponent, your words can be considered gold even if the vast majority of them are worse than manure. So when, just to use an example, the in-the-tank-for-Dr.-Pantsuit Washington Post publishes an article confirming that Dr. Pantsuit paid for the infamously bogus “dossier,” and in the course of doing so tries to promote the narrative fiction that unnamed “officials” have “confirmed” some of the information therein (yeah guys, you can confirm that Vlad Putin is the president of Russia, good for you), it is entirely in keeping not just with the legal rules of evidence but also just plain good sense to consider them more trustworthy by far on the stuff that goes against their interests. Of course they throw in the nebulous “but…but… collusion still happened! So said…um…officials!” to keep that story alive. Hell, the whole reason they probably ran this story in the first place is because the Russia thing is stalled out and they were hoping to jumpstart it with a little dose of truth in with their fiction.
Bottom line: you can believe one word and disbelieve the next. Courts do it all the time. It matters and it matters a LOT more whether the speaker is promoting his own interests. Pointing to the gem in the rubble doesn’t mean you think the rubble is anything more than trash.