Neither does the other side, unless you accept the premise that reading natural language text is similar enough to reading heavily indented code where that indentation has meaning. I don't accept that at all.
The studies that were done on natural language text assume that line length is equal to the column where the line ends (i.e all lines begin on the left edge). That's not usually the case for indented code.
If reading long lines takes more effort because your eyes have to travel further to find the next line, then the amount of indentation the next line has matters are great deal.
And looking at this before I hit go. You seem to ultimate be agreeing. Taking opening indentation into consideration concedes that long lines are bad. You just give some room for margins. And are ok with all of the mental stacking that the indentation implies.
Recipes often do two columns of some things. Math jargon is all about repetition. Narrative, explanation, exposition.
Don't get me wrong, I am inclined to agree with you. But we don't have a ton of empirical evidence on our side.