I think the 200 ms sampling period came from the need (or desire) to be able to operate the emonTx from batteries. Clearly, working for 0.2 s and then putting the processor to sleep for 9.8 s significantly extended battery life. And over time, that approach can be surprisingly accurate.
However, if you have a rapidly switched load, an induction cooker for example, then if the cooker cycles at (or a sub-multiple of) the 10 s sample rate, then you’re likely to always be sampling during an ‘on’ period or an ‘off’ period, so a large error might result. emonLibCM adopts a completely different approach and samples continuously, so removing that problem.
If you have mains power to your ESP32, there’s no need to take discrete samples once every 10 s. The longer the sampling period, the better the accuracy is likely to be. Sampling only current, you aren’t able to synchronise to the mains wave, and to get an accurate average, you really need to measure over an exact number of complete cycle, although where you start on the cycle doesn’t matter. Because you are sampling a lot faster than the '328P, you can afford to sample fewer cycles. You could sample 1 cycle (1623 samples) if you wanted, because one 1623rd part of a cycle error will make no real difference to the reading.
Now you know your sample rate, it’s clear why 1480 samples was a bad idea - you were not even measuring over 1 cycle of mains, you were missing about 10% of the cycle each time you measured.