I did some reading on the Life 360 app. It's been improved over the last two years, but just trying to get the basic idea. It triangulates satellite data to pinpoing location. In the settings, you can select how often it pings and communicates your location, say every 15 minutes. Did not find what the default setting is.
But the important part I learned is that the app sends out these location-identifying pings at intervals, which don't really drain the battery. It's not waiting for the family to send a call or query to respond with a ping. If the phone is on, it pings.
As ftl said, the free app that Charli had does not archive the pings. It only keeps the last ping in memory. And only the people designated as in the circle of life can see the pings on their phones.
So even though they thought the phone was still going, almost certainly it was not functional after that last ping at 10:56. Smashed, thrown in river, turned off never to be powered up again (or it would have pinged). While it would be possible I guess to go in to the phone and disable the app, I doubt much that he did it because it seems as if he forgot about the phones. Otherwise he would have dealt with his own iPhone to prevent it from showing his travel out there.
Maybe he remembered after the fact and that's why he admitted to going out there with her.
I too think the jungle near Paraquat's was the real dismemberment scene due to the hacking of the tree, because, as was said above, it was the perfect really remote place to do something that awful with little chance of other people showing up, little or no chance of being overheard either. The evidence was spread out to the point it was hard to find, and was not a good fit for a staging. There was no reason to even walk into that mess other than the app pinged it.
It seems unlikely to me that he knew the app was there and used it. The well known app on iPhone is Find my iPhone, that comes with the phone. He and Charli didn't hang out together such that he would know what all apps she used.