I think they are more concerned of disturbing it because the TNT itself is old. when it decays, (assuming the outer shell hasn't leaked) the Nitro glycerin starts leaching out, "sweating". that stuff is very unstable in that form, and can go off with the slightest vibration.
It's not the risk of it going nuclear they are worried about, it's the risk of it exploding and scattering uranium all over the place, making a big environmental hazard zone. they would prefer to let it sit there until the casing corrodes and leaks, the water will then dilute the tnt and render it safe. Then they can remove the core safely.
TNT has no nitroglycerin. It does not "sweat". Most dynamites "sweat", or settle out after prolonged storage if not turned. I doubt that TNT was used. Although I never had any "Restricted Data" access, I thought RDX or HMX was the component in PU fission devices. Overall, I suspect that sophisticated devices do poorly when stored in salty, high pressure muck. Since the AF says there's no trigger (that can mean many different things in the explosive train of a 3F nuke), the thing is about as dead as Franco.