I found the answer in the TeXbook source code (I hope this is not a violation of copyright, as the information is publicly available. If it is I will immediately remove my answer). I'm still dissecting how it works and learning TeX, so if anyone is willing to help explain that would be great.
\documentclass{scrartcl}
% from TeXbook source:
% https://ctan.org/pkg/texbook?lang=en
% /systems/knuth/dist/tex/texbook.tex
\countdef\counter=255 % define a new counter at register 255
\def\diamondleaders{\global\advance\counter by 1 % increment above counter every time this command is called
\ifodd\counter \kern-10pt \fi % says how far away initial dot can be, adds offset to kern (horizontal space) (?)
\leaders\hbox to 20pt{\ifodd\counter \kern13pt \else\kern3pt \fi % define a leader, still learning how these work
.\hss}\hfil} % hf
% ^(distance between dots) ^ these show the ^ relative displacement between the two ??
% from toying with the settings, I've found that adjusting 13 and 3 to the same value makes the dots a square grid
% I assume the 20pt is the spacing between the dots, and the 13 / 3 is some additional displacement if odd of even
\RedeclareSectionCommand[toclinefill=\diamondleaders]{section}% can repeat this for sub, subsub, etc.
\begin{document}
\tableofcontents
\section{Alpha}
\section{Beta}
\section{Gamma}
\section{Delta}
\end{document}

\leaders
): “The infinitely replicated boxes are lined up so that they touch each other, and so that, if you could see them all, one of them would have the same reference point as the smallest enclosing box.”