Congressional subpoena is really goddamn broad, and to a large extent just kinda' bypasses other clearance issues. As I understand it, clearance may vary from person to person, but a congresscritter doing congresscrittorial duties has what amounts to de facto clearance for anything a congressional subpoena can put in front of them. Anything, regardless as to if the person would have clearance under other circumstances. And a congressional subpoena can put just about sodding anything in front of them.
Or to put it another way, the extent it'd work out for the executive to hide shit that isn't seriously, seriously, beyond-just-getting-a-president-impeached seriously major is roughly shit fuck all. It wouldn't, and Congress would laugh at them and then maybe set their attempt on fire and piss on the ashes. It's one of those checksy balancey things, or something that arose from them over the years.
E: It's one of those reasons we really should be more careful about who we're putting in congress seats, honestly. Congresscritters may have been pissing around doing jack all a lot for the last while but they actually have quite a goddamn lot of power if they choose to excercise it. If.