Biblioarchy has commented here on what Oxfordians believe. He says " Bacon was not De Vere, and no Oxfordians believe this. Take my word for it, I know them." I do take his word for it, and it is a further objection to Eric Schwitzgebel's view that such belief ascriptions have an indeterminate truth value.
Nor do I see any difference between "The Oxfordian theory is that Edward de Vere was Shakespeare", which Eric agrees does have a determinate truth value, and "Oxfordians believe that Edward de Vere was Shakespeare" which, as I understand Eric, does not have a determinate truth value.
Biblioarchy's comment also reminds me of other verbs that create intensional contexts, such as 'is emphatic that', 'holds that', 'insists that', 'has received evidence that', 'has good reason to claim that' and so on. It is entirely implausible that the substitution problem connected with these verbs is different in any way from the problem of belief ascriptions.