Nor won’t – for the very simple reason that the processor (the '328P at least) can’t avoid dropping samples or incoming bytes when it has to handle each incoming character separately, along with sampling 30 - 40 samples per 20 ms cycle across the 3 ADC channels of the emonPi, and process and send the data onwards to wherever.
And that’s ultimately why, in the absence of the chip shortages and as they mentioned, G&T were intending to go with a STM32 solution for the emonTx V2 and emonPi2, rather than being forced into the AVR-DB in order to have a product to sell.