Depends on how "transmit only" is done. If it's using the equivalent of UDP and no encryption of the data, then you're likely correct.
But if the "transmit only" has anything that receives data - even just ack signals for what's sent, let alone something to negotiate encryption - all bets are off. :(
You're thinking a few OSI layers too high. Any such radio communications from similar systems have one-way broadcast as the physical layer.
If you don't include a radio receiver, then there is no capability to receive a single bit, much less negotiate encryption; the system throws out a waveform and the rest of the world may figure out what to do with it if they manage to receive it.
But if the "transmit only" has anything that receives data - even just ack signals for what's sent, let alone something to negotiate encryption - all bets are off. :(