No, Trump could not have declassified it. That is not how it works. Trump had the system explained to him when he came into office. The entire admin and all the workers are carefully told about how classification functions. As normal, Trump is lying. There is a procedure that includes notifying all the agencies that deal with the material so they can explain to their workers what is classified and what no longer is.
In fact, any info about nukes, including design, manufacture, number and utilization is designated "restricted data". That process is overseen by the head of the Department of Energy.
Any other declassification is done in consultation with the other agencies who have an interest in it.
It is not done by abracadabra methods.