In the tex version, I see (row 251 of unicode.tex)
(string-ci=? "$\latingreek{XAO}\Sigma$" "$\chi\alpha{}o\sigma$")
and \latingreek is defined on top to be:
\newcommand{\latingreek}[1]{\texonly\mathit{#1}\endtexonly\htmlonly
\textit{#1}\endhtmlonly}
This command produces latin letters not greek letters as far as I can
tell.