Mark space ratio in 1531 data signal

Maybe it's not just the 1531 but I just wanted to be specific in case
the older equipment is different.

https://www.luigidifraia.com/c64/tapwav/technical.htm

Here it says : "All these pulses are square waves with a mark space
ratio of 1:1" about the pulses C64 uses for the saved stuff on tape.

My question is simple : what is the significance of a full period
pulse signal's mark space ratio being 1:1 when actually zero crossing
of the signal matters?

Regards,

Nejat