Why isn't Courtney Crimsen's testimony likely true simply because it's against her interest?

Courtney Crimsen is incriminating herself, ruining her own reputation, exposing an embarrassing truth about herself, etc, and that is why her testimony is very likely true.

Admissions against interest lend weight to testimony they don't completely validate it.

Just because someone is apparently making admissions and statements that are, seemingly, selfless or embarrassing or incriminating doesn't automatically make them true.

The person making the statement might have deeper reasons or be guilty of even greater crimes than they are admitting and so would still lie.

All in all, it's about credibility.

Is her current testimony true...probably...but that's why we have juries...to decide.

The question has been answered but I wanted to append examples of how the proposed assumption is easily exploitable:

• Mob boss on trial? Get an underling to testify that they did it. Mob boss walks.
• The parent of a murderer doesn't want to see their child go to jail? They can claim they did it and basically exonerate their child.
• I'm on trial for a felony? I'll testify that I was committing a misdemeaner on the other side of town. If you assume that this testimony (against my own interest) is true, they you must invariably also agree that I couldn't have been there to commit the murder.
• Old or terminally ill people would be paid to testify that they committed the crime. They don't have long to live (and be punished) anyway, and this way they can leave money for their family.
• Just because a witness testifies to something being true, doesn't mean that the testimony itself is enough to arrest and convict the witness.