I agree it's probably a mistake but I can also see another possibility:
But first, consider the CCP. The CCP has nearly 100 million members. That's a lot of people. More than many countries. It's not a very exclusive club. Clearly such a large organization cannot be considered as a united whole. It's not just whether "the CCP can read it" it's about which part of the CCP can read it.
Can the low ranking CCP member read the wechat message of the high ranking member fucking his wife? Maybe not? But maybe he would like to? Maybe he knows a mathematician that can help him for a reasonable sum of money? Or maybe someone wants to do a bit of corporate espionage?
In other words the inner core of the party wants nobus, whereas the periphery has incentives to undermine it.