Here’s the relevant lines from the speech (emphasis added):

the law requires the President, or his designee, to certify that the suspension of sanctions under the deal is “appropriate and proportionate” to measure — and other measures taken by Iran to terminate its illicit nuclear program. Based on the factual record I have put forward, I am announcing today that we cannot and will not make this certification.

I deliberately said “decertify,” because that is the choice under INARA, rather than “withdraw.” Perhaps your confusion stems from interpreting the word “certify” in a sense that is different from how INARA, the president, my article, and others discussing this issue are using it.

For example, here’s FOX News: “Trump decertifies Iran nuclear deal.”