My guess it that they were doing a "two birds with one stone" strategy - using this project as an excuse (and test-case) for the translation layer, hoping that some devs would take this opportunity to port their DX9 games to Linux because of it, thereby improving the value of SteamOS.
Another option is that they didn't write DOTA2 from scratch, but reused an existing engine. Which in turn was based on some previous works, and at some point Direct3D was used, and remained there the whole time.
The air gap is not the solution. Proper isolation, firewalling and virus/malware is.
No. Firewalling, virus protection, malware detection... all these techniques can be flawed, either by design, because of oversight...
It is acceptable for most system (because these issues get fixed after a while), but for a SCADA system you don't want a zero-day to be exploitable *at all*. Your system can have a ton of backdoor/vulnerabilities/exploits, if it can't be reached by any other mean than physical access they are not an issue.
Serving coffee on aircraft causes turbulence.