What is more 'tool using' is carrying them around for use as a shelter later. And too, they modify them by blowing the mud out of them.
Here's another example:
This little octopus (that's a 1" shell) is in the process of taking shelter in his two-piece scallop-shell. Before being alarmed by the photographer, he was seen walking on the ocean floor, like the coconut-shell fellows, carrying his shell halves. He grasped one under his body, and one on top. To use his shelter effectively, he has to keep the two pieces oriented correctly, so they fit together tightly.
(source:
Octopus Tool Use? RAD Thoughts)
This example suggests to me that possibly the larger octopeds, who hide in coconut shells, are taking advantage of a new material to continue behavior they used as juvenials, but in the past had to abandon as they got too big for easily-found shells. Humans discarding split coconut shells in the lagoon created a new opportunity.