I suppose this stage of the rocket that fell to the ground during the launch of the satellite or something like that. I.e. it's just a large piece of metal.
From the article:
A chemical plant in Biysk, the largest city in the area, used to fill them with rocket fuel and after the defense ministry cut its orders in the early 1990s, the plant had plenty of such tanks stored.
The tanks were probably stored on the ground and washed away by the flood
But this one was "stored " a bit differently
“The Biysk plant uses such rockets, and when they are no longer needed sells them out,” Tatyana Zhdanova, whose family owns the tube, told the website. “Ours got away during the flood. Broke the fence and floated away along with the dogs, who were hiding inside.”