Money is like water; it will find the cracks and will flow through them. As has been rightly pointed out, the banking system, whatever one thinks, has spent a lot of money and a lot of time trying to seal the formal financial borders.
You don't move money through the formal financial system if you want to get it from A to B in an illicit way; you move it through remittance companies; you move it through bulk cash, as has been mentioned; you move it through trade-based mechanisms.
It's very important that one acknowledge that terrorist financing changes according to geopolitical situations and that it will change the direction it flows in depending on the barriers it faces.