Hi,
I can confirm that Apple watch does not support creating Bluetooth peripherals. There’s no problem connecting sensors, but you can’t get it to transmit data like a heart rate monitor would. I don’t know what the reason is, but I suspect it might be due to power consumption. Anyway, this restriction doesn’t apply to iphone apps, which is why you have apps that first transmit the HR data to the phone before being retransmitted as a peripheral that you can connect your head unit to.
I recommend you check out the Navigr8 pro app. It has this feature, but is also a fully fledged cycling computer. I actually ride with just Navigr8 running on my watch. I can easily connect my speed and cadence sensors and power meter to it and get my heart rate from the watch. No need for anything else😁