Unfortunately, the privacy model of an IRC private channel is only as private as the privacy precautions taken by members of the private channel.

Someone could run a ZNC bouncer on their own server, and have it connect to Freenode. Then, they could connect to their own bouncer using the IRCCloud web client - effectively gaining access to Freenode via IRCCloud, and Freenode would never 'know' IRCCloud is 'listening' in on the channel.