She did sign that letter. But she had previously said they did have the affair (and described it in detail). And shortly after she signed that letter, she admitted to Anderson Cooper the letter was a lie. She explained that she lied because… Get this… She was concerned about legal complications because she had accepted the hush money to not reveal the affair.
She also admitted that statement was a lie during her recent testimony while under oath. Signing that statement was not likely a crime. But if she had lied under oath this month, that would be a crime.
So, she was clearly lying at some point. Why do you choose to believe the much less plausible option that she was truthful the one time in 2018 but she was lying all the other times? There’s no logical explanation for that, yet the opposite (lied in the 2018 statement you posted) has much more logical support. The only reason I can see to believe her only that one time when she had reason to lie is because it lets you believe Trump’s lies that you really wish were true.