That isn't in-band signaling, that's encapsulating one protocol in another.

Then it is in-band signaling.

Out-of-band signaling is using a different channel entirely, like a different pair of wires, or otherwise a different interface.

Any system where there is a strict separation between data and control information is not considered a form of in-band signalling but rather out-of-band signalling based on an analogy of separation of a carrier into independent communication bands (using time or frequency division multiplexing for example). Many modern transport protocols can carry fully independent communications channels that way using frame division multiplexing.

But then, don't you have the problem of synchronizing the data channel and the control channel?

Well as an example you could have a sequence of messages, and every message starts with some metadata like what type it is (data / control) and how long it is.

It is possible to sequence messages across multiple channels although of course you have to design the transport protocol to support that. Alternatively you can just have typed, ordered messages within the same channel, which is a much more likely design for something like this.