I think your underestimating how much people hate Muslims in Europe, fear and bigotry drove people to vote Trump, even those who would traditionally be on the left... seeing as Europe has a bigger problem with the inclusion of immigrants and the migrant crisis, I would say the "brown men are here to rape and kill our white women" narrative has been built up sufficiently to allow Le Pen a solid chance.
First of all, Fillion also has a common theme of wanting to get the islam crisis under control, which tend to steal voters from FN, but if you think the left is gonna vote FN, you're insane.
They stand against literally every single value they have.
Muslims are not exactly a typical minority in France, pure "arabs" and muslims might have been decades ago, but many of them married french people and their kids that are now mixed are extremely numerous. They're the future, and even if they are not "muslims" themselves, they have family that is, and friends that know that, so there aren't as many racists as you think outside of old people.
There is a GENUINE concern though, that all the terrorists were french and muslims, so there is a problem in terms of security, integration and education, that needs to be looked at, and far more than what the left has been doing. But Fillion has addressed this.
A concerned left voter has no reason to vote Lepen over Fillion.