Basically: probabilities and credences aren't the same thing. They're measured on the same
scale, but probabilities can be used to reason about either facts or evidence, while credences are only "about" evidence (though potentially evidence
about facts.)
To put that another way: it is "100% true" that 2 + 2 = 4 (in ZF set theory); just like it is "100% true" that the sun will rise today, because it already did (and otherwise the Monty Hall problem would have a different answer.)
But the credence in an agent's mind can and should only be informed by observations of these facts; and the "observing hardware" cannot be 1.0 trustworthy. (Your senses can be imprecise; the brain can be buggy or biased; the environment being observed can be the construction of a Cartesian "evil demon"; etc.)
Given an artificial agent with perfect knowledge (e.g. one who is able to directly entangle itself with the environment it is trying to predict, rather than just one able to entangle itself with evidence about the state of an environment), some things can theoretically be held to have 0.0 or 1.0 probabilities (e.g. events that already happened)—but given an embodied agent, the "maybe my processing is flawed" argument should still make these into non-infinite credences.