It's not about measuring peak to peak in distance, it's about measuring how long it takes for one spot to encounter second peak after first. The fact that the first peak traveled some distance is irrelevant, as its entirely dependent on propagation speed, which doesn't affect the frequency, only vawelenght.

Would you then say that the wavelength is meaningful for the sound example as its properties are really of a wave propagating, and meaningless for the light as the wave analogy isn’t a full description of the light phenomenon behavior ?