there is only a "theorem" datatype 

You mean that there is NOT only a "theorem" datatype? In contrast to Curry/Howard provers, the LCF approach forgets proofs, it guarantees soundness by giving you access to proof rules only through the "theorem" datatype (which is the key trusted computing base). To be sure the "theorem" datatype may internally maintain a proof object (e.g. for the purposes of program extraction), but that's not necessary.