Text below equation with hf-tikz

\documentclass[aspectratio=169]{beamer}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{mathtools}
\usepackage{hf-tikz}

\begin{document}

\begin{frame}
\frametitle{Text under equation blocks}

\[
a = 
\onslide<2->%
  \begin{array}[t]{@{}c@{}}
  \tikzmarkin{a}\onslide<1-> b\onslide<2->\tikzmarkend{a}\\
  \scriptstyle\text{\alert{text1}}
  \end{array}
\onslide<1->+ 
\onslide<3->%
  \begin{array}[t]{@{}c@{}}
  \tikzmarkin{b}\onslide<1-> c\onslide<3->\tikzmarkend{b}\\
  \scriptstyle\text{\color{brown}text2}
  \end{array}
\]

\end{frame}

\end{document}