they do use bluetooth. However, it should be noted that not all BT devices are created equally. Check out this table from RTINGS.com of reviews of wireless bluetooth headsets. You can see that the very worst headsets have 300+ milliseconds of latency, while the very best have almost 0 ms of latency. I imagine that the Joycons hit a similarly low latency.
they do use bluetooth. However, it should be noted that not all BT devices are created equally. Check out this table from RTINGS.com of reviews of wireless bluetooth headsets. You can see that the very worst headsets have 300+ milliseconds of latency, while the very best have almost 0 ms of latency. I imagine that the Joycons hit a similarly low latency.