Seems he says a lot (really long videos) but then not so much to someone who is familiar with the science and engineering. Bottom line is that the useful capacity of a lead acid battery depends on how fast you take energy out! Wherever it goes is not pertinent since it is not accessible to the load. Batteries are chemical reactions and have time constants and nonlinear rate variations, which is all the Peukert said. He points at curves from Trojan, seems to disagree with them and then says they represent the "useful capacity" of the battery...which is what all of us are interested in. Most battery manufacturers state that the capacity figure comes from a measurement that runs down to a totally dead cell, not an inverter shutdown level.
His 1 + 1 = 2.7 undoubtedly represents the same phenomena that Peukert refers to but is only one measurement, stopping at the inverter shutdown voltage and perhaps measured in different terms. In addition, those batteries BTW are really strange AGM units that have a 2 year "shelf life." Not surprising that they might have a different discharge coefficient than a standard deep cycle flooded cell.
Seems the world is still round. More "useful capacity" if you discharge slower than the 20 hour rate, less if you go above that amount. Parallel 12 volt will cut the load in half, but equivalent 6 V units have twice the 20 hour rate anyway, because, they have roughly the same volume and weight and half the voltage so therefore twice the current resulting the same energy per pound or cu/ft.
Agree that this is fun and instructive, but we are probably losing most of the other posters with this "how many pixies can dance on a battery terminal!"