I will try to answer this as best as possible. There are a few factors that come into play here (Mainly the manufacturer of said engine).
Direct injection petrol engines work much like a diesel. You have a low pressure fuel pump that resides in the fuel tank itself, and a super high pressure pump that sits near the fuel rail that delivers fuel to the injectors. The high pressure pump is the first way fuel "Blow back" is avoided. It sits at a constant high pressure, which is MUCH MUCH greater than that of the combustion chamber. We know that if pressure of one atmosphere meets another and it is greater; Any fluid substance will not be able to pressurize. SO, being that the pump is pumping faster, and greater pressure that fuel should not be able to back wash into the injector theoretically.
The second step in this "Sealing" process is the injector itself. The injector has massive solenoids that allow it to open and close with great force. There should be several high strength rubber seals and a giant magnet. Unlike a normal solenoid these types of injectors can actuate both ways. Typically a standard fuel injector can only receive power to OPEN it. In a direct injection engine, they can be both opened and forced closed.
It's kind of hard to answer the question VERY thoroughly because it's not really that complicated, but if you want to ask this on an engineering forum that would probably be good too.
What it all boils down to, is that all of the seals and systems were engineered to handle it. There are MANY different types of these injectors as well, so it might be specific to that car. I hope I gave you a little insight at the very least. Check this video out too.