As per INARA, those are the same thing. The president certifies or decertifies the deal every 90 days. Technically, the law asks the president to certify both that Iran is complying and that “suspension of sanctions” related to the nuclear program is “vital to the national security interests of the United States.”

Trump emphasized that second part when announcing decertifcation.

If you mean that Trump has not formally withdrawn the United States from the JCPOA, that’s correct.

But the legal meaning of the word “decertify” in this context is specific.