"But they said after that we can consider CG as Pattern because E has already been used in another pattern "
you can't use CG if E was used in another pattern, you can only use it if E was used in that same pattern like in their example.
you can do this pattern: D-E-G-C
but not: C-G

...because those are only starting points, so E hasn't been used yet, maybe?

what? we used C->G. Just look at the picture in the description.

edit: well, it's G->C here, but that's just the same logic.

you cannot do a direct C->G if E is free. But if you used it (E) before, then you can.