DC current through diodes in AB amplifier
Yes, that equation assumes Ib=0.  We all know that can't be true IRL, but if the transistors aren't in saturation, it's likely that Ib is far lower than the bias currents used in the resistor/diode network.  So the estimate isn't a perfect solution in the real world, but it's close enough...and engineering is never about perfection, it's all about being close enough.