Airplane mode + 10k battery lasts ~6-12 days, depending on the person and the phone.

There are roughly 1 or 2 thousand attempted "thru" hikes per year, who hike from southern to northern border of the USA over the course of 3-5 months. Far more "section" hikers, who will do a 1-4 week segment of a larger trail.

Such folks often resupply food and recharge their battery banks every few days.

To be fair, we'd generally call it backpacking rather than hiking. But the premise of the article was backpackers, so I didn't bother distinguishing.