You mean that:

% Set the textarea width to 2.6 alphabets.
\setbox\scratchbox\hbox{\dorecurse{26}{\character\recurselevel}}
\newdimen\Width \Width=\dimexpr(2.6\wd\scratchbox)

That's something I don't understand! What is a 2.6 alphabet? What means "scratchbox", "2.6\wd" ... ?

This is straight TeX (not the higher-level nicety of ConTeXt). What it does is create a throw-away box (a scratchbox) and sets in it horizontally one lower case alphabet of the current body font. It creates that alphabet by setting \character{1}\character{2}...\character{26} using a loop. It then measures the width of the box holding the 26 characters (\wd\scratchbox) and multiplies that by 2.6, depositing the result in \Width.

A more ConTeXt-y way to do this might be:

\def\Alphabet{abcdefghijklmnopqrstuvwxyz}
\setupbodyfont[libertinus,12pt]
\setwidthof\Alphabet\to\AlphabetWidth

and then use width=2.6\AlphabetWidth in your layout

--
Rik