Yes, this is missing the "pressure" part of "backpressure", where the recipient is able to signal to the producer that they should slow down or stop producing messages. Observability is useful, sure, but it's not the same as backpressure.
Sending message to a process has a cost (for purposes of preemption) relative to the current size of receiver's mailbox, so the sender will get preempted earlier. This isn't perfect, but it is something.