I suppose I am interpreting the graph incorrect but doesn't it show that the rising edge of I(d6) at 30volts?
Yes, if you look the pulse is centred around the zero crossing, with the component values chosen to make the pulse width ~1ms. It has a predictable and consistent delay (subject to the usual factors that effect drift in any analogue circuit, tempco of components etc.). One can adjust the component values to make the pulse narrower, subject to the capacitor holding enough charge to adequately drive the LED in an optocoupler.
That it comes
before the zero crossing is no particular issue, a circuit that triggered exactly at the zero crossing would in practice, because of propagation delays, deliver the zero-crossing indication
after the zero-crossing. Having the signal trigger slightly beforehand is actually an advantage for many applications - if you want to switch something
at the zero crossing it's more useful to know that the zero crossing is coming up in ~500us rather than has just departed 100us ago. If you're using the zero-crossings just to measure the frequency of the mains you don't care about the delay either way as it's indication-to-indication timing that's of interest.