It is categorically false that BB(748) is not computable. On the contrary any particular BB(n) can be computed by some Turing Machine even though there is no Turing Machine that can compute BB(n) for every n.
A proof of existence is not the same as a construction, so the fact that we know that there exists a TM that computes BB(748) does not mean that we know which specific TM does it or how to construct such a TM.