Pipe just means tube, not that you have to use it as a fluid vehicle. There is structural Pipe. And they said they pumped the water out of it, which then let it float up on a presumes rising water table below, meaning it was hollow inside.

From JapanTimea article:

The pipe was being used as a retaining structure to prevent excavated soil from collapsing. Workers had been pumping groundwater out of the pipe until early Wednesday morning, but no abnormalities had been detected before the incident.

Support pipe use, like when you dig a hole and shore it up on the sides. Rathen than a pile driven in to stop layered geology from aliding off a slope. The articles mentioned they were using it to get to the bottom and do their connection work...not connect too this pipe, but it can still be called a pipe by definition of what piping is.