I described their decision in the Post Pay article in the first issue. In the second issue I quoted Bell System literature that detailed how concerned they were with this problem when they re-introduced dial tone first in 1968. My speculative argument assumed that ground start is quicker than loop start. It does not mean necessarily that ground start is faster. That's a chief reason for their use.
Ground start is a DC signal just like loop start.
Most pay phones are within three to eight miles of a central office. All DC signals must act as if they are instantaneous. Any difference in time between loop start or ground start is probably minuscule or irrelevant or both. That's not all. I used two other facts to bolster my argument that pay phones used ground start.