I’m not sure if they are aware of Nostr.

What I see is that their underlying relay protocol is similar to Nostr publish subscribe.

https://github.com/simplex-chat/simplexmq/blob/stable/protocol/simplex-messaging.md

That means someone should be able to reimplement SimpleX duplex channels over Nostr relays:

https://github.com/simplex-chat/simplexmq/blob/stable/protocol/agent-protocol.md#communication-between-smp-agents