Legal Question in Criminal Law in Texas

Yes. The prosecutor has the burden of proof. Defense counsel can challenge the prosecutor's evidence. She can introduce evidence that undermines the prosecutor's. She can (and must) argue that the prosecutor failed to carry his burden. She can make legal arguments that the charges are defective or that the prosecutor has not followed proper procedure. There are other legitimate ways she can advocate for her client. She just can't introduce perjured evidence.

Note that sometimes -- more often than you'd probably think -- people who say they committed a crime actually didn't. Often, it's because acts they think were criminal really weren't. Sometimes it's because they're mentally ill. Sometimes they're lying to protect a loved one. Sometimes an aggressive interrogation just wore them down. There are other reasons, too. This doesn't mean counsel can presume a confession is false. But it does mean the confession's existence does not prove it is true.