SOLVED
THE GAME IS LISTED AS NEEDING OSX 10.13 AS MINIMUM HERE.
So... yeah.

INSIDE was not available for Mac on launch, and for a long while it remained so. So long that I forgot about it. One or two days ago I found out there were already Mac installers available (who knows for how long) and quickly downloaded them and installed my game.
And after waiting ages to play this on my Mac, I get this cruel error when launching the game:

(Something something error name): No suitable Metal graphics device found to run INSIDE.

I hadn't the slightest idea what "Metal" was so I search around and found that it is what has been replacing the OpenGL graphic libraries on newer MacOS (from a couple of years back).
I then tried finding out if my machine was NOT compatible with it and found that in fact, my Late 2012 computer running Sierra, IS compatible ( according to this list at the official website: https://support.apple.com/en-us/HT205073 ).

So my question is rather obvious now: What the what?

Another question just sprang out though, "how do I make the game work?"
There is nothing wrong with my computer or OS, so in theory all should be fine.

If anyone has had this same problem and knows of a workaround, please advice.

Thanks a lot for any info.