The apple watch ECG corresponds to lead I. Lead II and III is right and left arm respectively to left foot, so they will be possible with a bit of flexibility. The 6 precordial leads are not possible, since they are measuring the voltage between an electrode on the chest and the mean of the 3 extremity electrodes.

The poor man’s single lead ECG is complemented by continuous PPG heart rate data. Being able to combine the data of multiple sensors that are synchronized with a single clock provides new opportunities for monitoring and analysis. HRV data is also useful and I’m surprised that it hasn’t been used to estimate the respiration rate.

Quite unlikely they did not try. Reflectance pulse oximetry is unfortunately affected a lot by ambient light and so, even for the Apple Watch, that might have caused unreliable measures.

Using the ECG for RR estimation though should be more than feasible...

I have very little understanding of how an ECG works. Can you ELI5 why a single-lead ECG recording can be off by 30 seconds, and what that means?

He means that you would have to do many single-lead recordings separately, rather than a multi-lead recording once. The standard length of the Apple Watch ECG recording is 30 seconds.