The problem with that theory is that we know exactly what Roman drop spindles looked like. Spindle whorls are one of the more common artefact from the Bronze Age on, so every archaeologist working in this area and time period is familiar with them. And unfortunately this isn’t what a Roman spindle would look like. It was much narrower, the weight being provided by a stone or pottery (or in the case of the Romans, often lead) spindle whorl, rather than by the extending width of the wooden shaft.
I have no idea about darning, whether the Romans used it and which tools they needed for it. One would have to ask a textile archaeologist for that. But as I said, rather than being obscure, as the layperson commenter in the Guardian seems to think, spindles are the bread and butter of European archaeology, one not being recognised is unlikely. I still lean towards pestle for this artefact, but if darning was a thing in Roman Britain, I can see that as well.
ETA: I just saw the comment of an eminent textile archaeologist on Facebook. This is all she said: “How in hell is that a drop spindle??!!”
ETA2: She continues “cannot imagine spinning with this huge thing. Your yarn and spindle would flop around like a flat tire 
”
