This deal was made well before Singapore happened and was primarily with China and Russia. Donald Trump really isn’t stupid.
He was pretty darn naive through much of the campaign in regards to Russia. This was before he received some badly needed "schooling" by some wise, newly appointed folks in his administration, particularly his VP pick, Pence. He said around this time in an interview, [pp] "Believe me. I learned a LOT in the past two weeks". I believe the subjects of the conversation was foreign affairs and foreign policy.