Yep it does say that but i was thinking may but that would only be if you actually want the keep_alive signal to work. It doesnt matter any way as i have looked at the code now and it checks to make sure the value is less, and if not corrects it.
I cant see a way to disable it just though the config but to prevent the keep_alive from working at all set it to false, and also set min_keep_alive to false. You then also need to add keep_alive to dont_override to prevent the user from overriding this setting. Finally in program/js/app.js comment out "this.env.keep_alive = 60; // seconds" (line 59 in my SVN copy)
This will prevent RC from sending any keep_alive or check_recent messages and cause the webmail session to expire after what ever session_lifetime is unless something is done by the user. It will also prevent the automatic checking for new messages.