You're excluding the part where:

A) it was only partially destroyed

B) Russia offered to run gas through the remaining pipeline at a higher price, almost immediately after running campaigns pointing fingers at everyone else

Apparently with A whoever did it decided not disable it completely, and with B somehow Russia is extremely confident it won't be attacked again.

It's not a hard conclusion to reach that Russia bombed its own shutdown pipeline, while leaving it functional, in order to cause confusion amongst enemies.