查看 abstract 宏包的环境定义，可能和内部调用 center 环境有关
\if@titlepage \renewenvironment{abstract}{%
\titlepage \null\vfil \@beginparpenalty\@lowpenalty
\begin{center}%
\bfseries \abstractname \@endparpenalty\@M
\end{center}}%
{\par\vfil\null\endtitlepage}
\else \renewenvironment{abstract}{%
\if@twocolumn
\section*{\abstractname}%
\else \small
\begin{center}%
{\bfseries \abstractname\vspace{-.5em}\vspace{\z@}}%
\end{center}%
\quotation \fi} {\if@twocolumn\else\endquotation\fi}
\fi将 MWE 中的 abstract 环境改为 center 环境
\documentclass{article}
\usepackage{showframe}
\usepackage{lipsum}
\usepackage{abstract}
\setlength{\absleftindent}{0pt} %取消左边距
\setlength{\absrightindent}{0pt} %取消右边距
\renewcommand{\abstracttextfont}{\normalsize} %确保摘要环境内外字号一致
\usepackage{enumitem}
\setlist{
itemindent=\parindent,
}
\begin{document}
\begin{center}
\lipsum[2]
\begin{enumerate}
\item this is text. this is text. this is text. this is text. this is text. this is text. this is text. this is text.
\item this is text. this is text. this is text. this is text. this is text. this is text. this is text. this is text.
\end{enumerate}
\end{center}
\lipsum[2]
\begin{enumerate}
\item this is text. this is text. this is text. this is text. this is text. this is text. this is text. this is text.
\item this is text. this is text. this is text. this is text. this is text. this is text. this is text. this is text.
\end{enumerate}
\end{document}
效果和以上展示的一致，初步判断就是 center 所致。
问 enumitem 宏包 itemindent=\parindent 的设置在abstract环境内、外的效果不同？