I must not fully understand your dilemma, perhaps if you posted some pictures. I assume the IP-256 is similar to other Chiner frames in that the internal cable routing, or a some people call them, cable liners, are not meant to be removed. If you ever change out your cables or brake lines, you will need the internal routing to help with installation of new cables or lines.
I have yet to see a Chiner frame that does not allow internal routing of a brake line, in the holes routed to the rear caliper. If that were the case, nobody would purchase these frames. If you chose to do things differently than they way the mfr intended, then you do so at the risk of not having proper shifting. Not saying it won't work as I do not own this frame and maybe what you want to do is fine, just saying that's not typical for these frames.
Shift cable housing, and brake lines are two different diameters, ~4.25 for shifter cable housing, ~5.0 mm for hydraulic brake line. I had zero issues with my IP-057 cable & brake routing. My IP-033 was a bit different for the rear derailleur, I ran a full cable housing from the shift lever, through the frame, to the rear derailleur mech. For the IP-057, I cut the cable housing at either end of the frame and installed a ferrule, so there is bare cable inside the frame.
I believe the IP-256 is like the IP-057, bare cable inside the frame for the front & rear derailleurs, and for the rear brake, the internal routing is sized to fit a hydraulic line and designed for the standard (at least in the US) right-rear brake lever setup.
UPDATE to this post:
Looking at the IP-256 on the XMIplay website, you must be specifically referring to the f/d cable routing only. I see a piece of inner cable guide sticking up out of the frame. Typically, for the front derailleur, you would run bare cable through the frame, out the cable guide (liner) and up to the f/d mech. For your dropper seat post that you instead want to use this routing for, I suppose you would need to do something different as you probably do not want to see bare cable all the way up to your dropper seat post. Since it was not designed for the dropper seat post, you will need to figure out how you want to do the cable routing. As you said, the diameter of the f/d cable routing is not sized for a hydraulic line so your only choice is cabling.
I might first try cutting down the extra liner, and running cable housing from the frame up to the dropper post with a ferrule at each end. Hopefully the ferrule does not slip into the frame. If it all stays in place, the cabling should work up to the dropper post.