Once a judge has ruled that something is irrelevant, it is not coming in, no matter what. It doesn't matter if it's authenticated, un-privileged, competent, fitting within a hearsay exception, etc. It is not coming in. If it is irrelevant, it is out, and nothing else matters.
That's why we're spending so much time on Article 4: relevance is the cornerstone of evidence.
So, if something is relevant, there's a basic presumption that it can come in-- there are a few restrictions, and we can think about general reasons why relevant evidence might be excluded:
This defense ("I didn't do it, and I know who did") is a common one. In WI, this is a Denny issue (State v. Denny Ct. App. 1984). Courts are leery about this kind of evidence: it's very powerful, and potentially very important, but it also carries lots of potential ill effects/risks. Basically, courts are concerned at least in part about the reputation of the 3rd person, who is being tried, in effect, in absentia, without an advocate. And this might inappropriately sway or inflame the jury. And also, it might be possible to admit all sorts of evidence against this third party which wouldn't be admissible if that person were the defendant.
So states have (in case law) multi-prong tests for when there's a strong enough showing of third party evidence to allow it to be admitted in trial. There's a preliminary test, in other words, before you can assign blame to a named third person at trial.
And so, in SC, the analogue to Denny is Gregory.
The trial court says that the third party evidence is irrelevant, because the opposing evidence is so strong. That, of course, does not make the evidence irrelevant. Rule 401 tells us how to decide if something is relevant, and it's just about what is being offered by the proponent-- not what is being offered by the opposing side.
The court has made a determination of truth, not a determination of relevance, and that's the error, at least initially.
The Rule 403 problem, then, is that the probative value of this evidence wasn't substantially outweighed by any of the factors listed.
It's OK to have a threshold standard for third party evidence, but you can't just exclude it because the other side's case is stronger.
Relevance is decided without reference to the opposing party's evidence.