pablodusk: Thanks. I don't know why they've had so much trouble getting their info right recently.
Strange indeed. In this case, they first updated the info for the deal to state:
"DRM-Free available on Windows. Mac/Linux files will be available shortly".
But a couple of hours later, that message was replaced by this one:
"DRM-Free version AVAILABLE FOR WINDOWS ONLY. Steam version available for Windows, Mac, and Linux."
Which is still there, so no Mac and Linux DRM-free builds after all. According to the email I've got from IGS' tech support, the dev had some issues with the Linux DRM-free build "due to instability" (whatever that means). I'm getting my purchase refunded.
PS: I've tried the Windows build with the native Linux AGS engine and it seems to work, so there's that. But who knows whether the game can be actually played throughout?