I need a trained eye. Which part of `\usepackage{tikzpagenodes}
\titleformat{\chapter}[block]%
{\relax\ifthenelse{\NOT\boolean{@tufte@symmetric}}{\begin{fullwidth}}{}}% format applied to label+text
{\begin{tikzpicture}[remember picture,overlay]
\node[anchor=north,inner sep=0pt,draw=gray,font=\itshape,text=black!10] at ([yshift=-147pt]current page marginpar area.north) {\resizebox{!}{5cm}{\thechapter}};
\end{tikzpicture}}% label
{0pt}% horizontal separation between label and title body
{\huge\rmfamily\itshape}% before the title body