you can have x "message" sent per round, where a message is a packet (a line can be split over multiple packets) and x must remain constant and in the implementation is 1.
now the problem is, the adversary modelled here is one that can drop a node out of the network and see if any other node message frequency drops.
that's why traffic, noise and latency are strictly correlated. if a conversation suddenly drop you have to make sure that the noise level doesn't drop, and vice versa.
when you start a conversation you need to tune down the noise, to maintain the frequency. thus the more noise you generate the more packet you can send when you need it, but the more the load on the servers.
it's not about hiding the message in the noise (that's tor model) it's about making messages statistically indistinguishable. for that there is no confidence interval, you need to send random noise out, randomly, at a fixed randomized frequency.
now I think they don't actually send a message every round, but do generate noise and messages interlocked so that the frequency remains constant. that frequency must be slower than rounds frequency and you cannot really tune it in the way you're suggesting. There might be probably other way to relax the confidence, but it might have impacts on other clients on the network, even those unrelated to your conversation.
also, under this round model, if a client drop after handshaking and before collecting the message the message for that handshake is lost (no retransmission and no chance to pick it up later)