the mod should use IC2's function and not reborncore. This might be due to unintentional classloading. (if you run a function all classes used within it are loaded whether that path is reachable does not play a role. just my guess is that you had an if to check which mod actually is installed but by doing so you accidentally loaded reborncore. forge 1.13+ has a lambda utility to avoid this you might want to copy that code if you do not mind using forges GPL license
the mod should use IC2's function and not reborncore. This might be due to unintentional classloading. (if you run a function all classes used within it are loaded whether that path is reachable does not play a role. just my guess is that you had an if to check which mod actually is installed but by doing so you accidentally loaded reborncore. forge 1.13+ has a lambda utility to avoid this you might want to copy that code if you do not mind using forges GPL license
https://pastebin.com/bMMWKgg8