The french two round system has come to their aid here. The initial intervention was the destruction of the previous mainstream front runner, the republican, Fillon. The most likely outcome was then a run off between Le Pen and a very weak socialist, Hamon, which she'd have had a good chance of winning. They were too late in trying to get to Macron to cause the kind of destabilization that they want because he did too well in the first round.
We'll never find out exactly who "they" are - but we should take note to build some checks into the political system to keep it robust against these kind of influences.