I've tried the sample provided in the linked mail:

\documentclass{article}
\begin{document}
[this is for test $a+b=c]$
\end{document}

It seems that I've fixed this issue in the meantime, no extra <mrow> elements are produced, the output looks OK:

<p class='noindent'>[this is for test <!-- l. 3 --><math display='inline' xmlns='http://www.w3.org/1998/Math/MathML'><mrow><mi>a</mi> <mo class='MathClass-bin' stretchy='false'>+</mo> <mi>b</mi> <mo class='MathClass-rel' stretchy='false'>=</mo> <mi>c</mi><mo class='MathClass-close' stretchy='false'>]</mo></mrow></math>
</p>

So I think we can close this now.