Is the set of language decidable by some Turing machine computing in some given computable time bound decidable
This is a situation where coding is non-trivial. $\mathcal{C}_T$ is a class of decidable languages. To make sense of $\mathcal{C}_T$ having any effective properties, we need to choose a coding. Naturally, we would code some decidable language $L$ by the index of some TM that decides it.

So what does it mean for $\mathcal{C}_T$ to be recursively enumerable? It means that we can effectively produce a list $(M_i)$ of TMs such that $\{L(M_i) \mid i \in \mathbb{N}\} = \mathcal{C}_T$. Note that this does not give us that $\mathcal{C}_T$ is semi-decidable -- that would mean that given some TM $M$ deciding a language, we can recognize that codes a language belonging to $\mathcal{C}_T$.

The familiar "semi-decidable = computable enumerable" holds only for spaces with decidable equality -- and the space pf decidable languages does not have decidable equality.

Furthermore, the complement of $\mathcal{C}_T$ is not semi-decidable, either: While we can recognize that a given TM takes too much time, we cannot rule out that there is another TM deciding the same language, but respecting our time-bounds.