Let me explain where I come from, idea-wise. Since the disappearance of Denise Thiem, I have taken an interest in all things Camino and Camino forums. Pilgrims on the Camino (and elsewhere) live in a bubble of sorts. They exchange info, they pose questions among each other. On the forums, FB and along the way. Someone will ask for instance if Albergue / guesthouse X is still open. One person will answer: it was last year when we were there in April. Another one posts that it was closed in December. And so on. Hopefully the person who was there last week comes along.
The same goes with cellphone coverage. Pattie could have gotten her info from someone who hiked the Kohechi trail at a time when coverage wasn't good. I still have the impression that the Kohechi trail was not part of her original plan. It was mentioned somewhere - cannot find it right away, sorry - that she was ahead of schedule and that is why she could do this extra 4-day trek. IMO the schedule is what she had planned in advance, and perhaps the Kohechi not so much.
For Dual Pilgrim Status, it would have been sufficient to hike the Kohechi Trail. But Pattie had hiked a much longer distance, on a different route.
More info here:
Dual Pilgrim|Tanabe City Kumano Tourism Bureau
On the trail, the pilgrims collect stamps in their Dual Pilgrim Credential (a sort of Pilgrim Passport) as proof along the way.