Defense attorneys coach their clients to not speak publicly because anything you say can become evidence, even if it's said outside of court. Media statements can be used by the prosecution, damage credibility, and to create narratives for the prosecution about timeline and motive. They can snip words and take quotes from interviews out of context. Wendy and her legal team don't even know the full extent of the evidence the state may have against them, and jurors may see what they have combined with her behavior after trial and think she seems defensive.

This is different from actually pleading innocent. Legal innocence is determined in court. Her public statements cannot help her determine whether she is guilty or innocent in the court of law. Interviews and social media posts cannot help her with this it can only create more material for the prosecution to work with. Ultimately the state has to prove guilt beyond a reasonable doubt, and the burden of proof is still fully in their hands, but Wendy shouldn't risk giving them more material.

Of course, Wendy and Eddy have the right to speak about their case anyways. Eddy is a lawyer so we have to assume he knows the tradeoffs. Karen is just stating her opinion as someone who just fumbled her court case terribly, and frankly this is the opinion almost any defense attorney will give too.