Tom is just repeating what he heard. I bet if you plug a 4.8v battery into one input and a 6v battery into the other you will see this voltage drop that he talks about, because the diodes are sensing the lower voltage and switching to run off of just the 6v battery. It takes a voltage drop on one side to see this - it can't just be an open circuit.
NO, Tom knows this because het tested it under load. Please refrain from telling people what I know and don't know and how I know it.
Diodes always cause a voltage drop with current flowing, that's where the term "diode drop" came from.
People use a couple diodes in line with the power to reduce voltage to their ignition when using lithium batteries.
TF