Another law report: I've now accessed the current US Espionage Act language in the current US Code, as opposed to the historic version I saw previously. This is 18 U.S. Code § 793 - Gathering, transmitting or losing defense information.
I abstract and summarize:
(e) Whoever having lawful possession of, access to, or control over any...information relating to the national defense, and transmits it to [someone who should not get it], or willfully retains the same and fails to deliver it to the officer or employee of the United States entitled to receive it, or
(f) As (e), regarding anyone with unauthorized possession, or
(g) Anyone, having lawful possession, etc, of any...information relating to the national defense, who (1) through gross negligence permits the same to be removed from its proper place of custody or delivered to anyone in violation of his trust, or to be lost, stolen, abstracted, or destroyed, or (2) having knowledge that the preceding has happened, fails to make prompt report of such —
Shall be fined under this title or imprisoned not more than ten years, or both.
https://www.law.cornell.edu/uscode/text/18/793
Note that the act speaks of "information relating to the national defense." The complete statutory language describes such information in considerable detail. It does not speak of information of any particular 'classified' status. Rather, it criminalizes acts (or failures to act) regarding national defense information as described, apparently regardless of whether is is "classified" or not. I believe this, if nothing else, defeats the defense that Trump had authority to declassify information.