Nowhere do I see where he said he used a scope (nor is that applicable), I suspect he measured the resistors with a DMM (and they are not accurate in-circuit). You aren't understanding me, the circuit he captured DOES NOT TIE LINES TOGETHER. Each output and resistor pair is a Thevenin source, and in parallel you get weighted-averaged MIXING of the sources. This is completely different than tying signals together, and no offense to tomaitheous but tying signals does not sum them nor is it smart nor is summing even what you want to do here since the composite circuit does not sum. Summing requires precision mixing (identical Thevenin resistances for an exact midpoint between the voltages) followed by precision voltage amplification.
I also don't see an analysis anywhere of the circuit but I can give a poor one. Y, I and Q ("R-Y" and "B-Y") are voltage mixed then further mixed with colorburst, in this order seemingly for economical purposes. This node is buffered with a common-collector amp and attenuated, then buffered with a complementary common-collector pair. I think the PNP is to negate the Vbe drop of the final NPN CC which sets the output impedance. Because the 75 R is shunt and not in series, and there is no voltage amplification, and there are no shunt resistors for the DAC output it can be assumed that A) the DAC is voltage-driven and B) the signals are directly ~1 Vpp. This still doesn't make it clear which values to use. Analysis of the DAC and transistors is difficult so it's likely even Hudson adjusted the attenuation for example with a scope.