"allegedly", or he did what everyone else does, tried to hire a service to do opposition research on politicians.

“They wanted to gather intelligence on senators, judges, congressmen, state reps, to move them towards the audit. The word ‘move’ was emphasized tremendously. It was clear to me what they wanted was not traditional opposition research — what they wanted was to extort and to literally move people towards the audit with dirt.”

So this "allegation" is based on Stern's claim of what "move" means. The "journalist" who wrote the hit piece didn't even bother stating whether or not they contacted the Flynn team for comment.

Fake news is always gonna fake news I suppose...