thats... not true.
Hard water will leave lime scale or the equivalent on your pipes if theyre metal (galvanized or copper), it wont leave anything on PEX or PVC/CPvC
Acidic water (often associated with soft, but they are two distinct characteristics not intrinsically tied, will mess with copper and galvanized pipes, but once again, not pvc/cpvc
If you've had hard water, and build up significant scale on your pipes to the point that its all thats holding them together, soft water may dissolve this scale, causing leaks in your pipes because they were at the end of their life anyway and were only being held together by the scale.
In no situation on its own will soft water damage your pipes.