Of course not, they merely have the power to declassify any documents they wish.
How can they declassify something they don’t know exists?
In the particular matter we’re discussing, Trump either a) knows they exist, or b) is making unsubstantiated shit up again. Because either he knows a factual basis for the claim or he doesn’t. If it exists and he didn’t know about it, that means he still made it up–he just got lucky by the stopped-clock standard.
Also, as Cheney taught us, there are other less-straightforward but equally effective avenues for declassifying info.