They don't have any proof (probably because it didn't happen). One of their staff clowns made a long post including video of the TV copy about the 8 minute mark of the second half and then circled in a still photo where they think Tang and an Assistant were pointing to the "spies". The pointing is more towards the corner area, 10 rows up maybe, not behind the bench. Pretty nice seats but high enough up I have no idea how a cellphone would get anything helpful audibly or visually.
They also deleted a post from a staff member about the "incorrect" theory ISU had a plant in security/arena uniform that was behind the bench and communicating the ISU staff. That likely occurred because Cyclone Larry screenshotted it and put it on Twitter and they got appropriately ridiculed. I chuckled because THAT post was too crazy but spying accusations in general totally make sense.
A couple of posters are correctly questioning how spying would be feasible given the time constraints and pointing out their staff and fans look batshit crazy without evidence. A few others have mentioned gambling or "that's how ISU covers spreads", and most just blindly believe it.