Perhaps the US/Israel discovered the prospect for this particular kind of software-based functional sabotage in defensive analysis of their own similar industrial systems. That analysis would have deserved vendor technical cooperation, perhaps even source code and engineering design documents, for wholly legitimate reasons, as a matter of course.

It shouldn't be surprising that any especially insidious risks discovered during that process could then be shared with the more covert and offensive branches. That wouldn't be Siemens' fault at all.