Mechanical injection should be available in the game in 1968, as it is in the Kee version of the game. I don’t remember why is it changed in UE4, but IIRC it’s not meant to unlock as late as 1978, with EFI. 300SL had mechanic direct injection, and it was just that one production car with such technology made in history (AFAIK), so it’s not a good example. Mercedes in particular was quite special with their early adoption of injection.
And, in addition to what @Awildgermanappears wrote, in the campaign (even the lite version available currently in the Kee engine) technologies can be unlocked up to 15 years before their normal unlock date through investing into research, so in an extreme case (which the 300SL definitely was) you could make a car with injection as early as 1953, and probably release it to the market in 1957-1959.