I am not a midi specialist, but the general idea is that to choose and play a sound from your midi keyboard, you have to send the corresponding "program change" and "msb" messages (these numbers are listed in the "apendix" in the manuals section under columns 4 and 5).
Either your midi keyboard can send these messages and you don't need your computer, or it can't and you have to send these messages through Logic Pro.
Anyway, the fact that these 2 messages are listed in the manuals should mean that a CT-X can be used just as a midi sound module.