Thats only true for Lmax, Fxcm and i believe CTS. Sierra has set up a dedicated FIX-Server for those. The advantage is that you dont have to meet very high minimums from these brokers. With Lmax for example you need to do 500 million per month in trading volume for a Fix-connection(or pay $2500), but not if you use Sierra. The latency is minimal since this sierra-server is proximity hosted at lmax. I dont see a problem here.
With other brokers like Interactive Brokers, trades go directly through the api. Same for TT, Rithmic etc.
With other brokers like Interactive Brokers, trades go directly through the api. Same for TT, Rithmic etc.