The universal semimeasure is "computably approximable from below", aka (lower) semicomputable, meaning that you can computably list out all the rational numbers below a given value assigned by the measure. The probabilities assigned by a logical inductor are computably approximable reals, which (confusingly) is weaker than lower semicomputable; it just means that you can compute a sequence of rationals that converges to the real, with a possibly uncomputable convergence rate.