What is the significance of first zero upcrossing of a cross correlation?
Not sure what you are asking specifically. The cross correlation of two cosine waves is just another cosine. The triangular envelope in the picture is simply a consequence of the limited length of the signals.

The phase of the cross-correlation cosine is given by the phase difference of the two input cosines. This phase difference determines the position of the zero crossings. So in effect the position of that zero crossing is some metric of the phase difference of the input signals