\documentclass{lms}
\usepackage{etoolbox}
\makeatletter
\AtEndPreamble{%
	\let\proof\relax
	\let\endproof\relax
	\let\old@thm\@thm
	\let\old@xthm\@xthm
	\let\old@ythm\@ythm
	\let\old@begintheorem\@begintheorem
	\usepackage{amsthm}%
	\let\@thm\old@thm
	\let\@xthm\old@xthm
	\let\@ythm\old@ythm
	\let\@begintheorem\old@begintheorem
}
\makeatother
\newtheorem{theorem}{Theorem}[section]
\newnumbered{definition}[theorem]{Definition}
\usepackage{thmtools}
\begin{document}
\begin{theorem}[({cite some people})]
This doesn't display as a theorem.
\end{theorem}
\begin{theorem}
This theorem displays fine. Why?
\end{theorem}
\begin{theorem}
	If $X$, then $Y$.
\end{theorem}
\begin{theorem}
Now
\end{theorem}
\begin{theorem}
a
\end{theorem}
\begin{theorem}
few
\end{theorem}
\begin{theorem}
more...
\end{theorem}
\begin{definition}
too deeply nested?!
\end{definition}
\end{document}