I suspect that two spaces make sense when you also limit the length of line to some maximum below 80.

I.e. I suspect that those using 4 or tabs also use long lines.

And long lines are not considered very readable these days...

As a result, my personal conclusion is that two spaces indented code shows a stronger interest in readability.

But I don't have numbers to back this claim and I understand it is contreversial. Maybe the author of this nice tool can add info about long lines?