LuaLaTeX + libertine... the issue of math font

Since you give the no-math option, you have to help fontspec:

\documentclass[]{scrbook}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathtools}

\usepackage[libertine]{newtxmath}
\usepackage[no-math]{fontspec}
\usepackage{libertine}

\DeclareSymbolFont{operators}{\encodingdefault}{\familydefault}{\mddefault}{n}
\DeclareMathAlphabet{\mathit}{\encodingdefault}{\familydefault}{\mddefault}{it}
\DeclareMathAlphabet{\mathbf}{\encodingdefault}{\familydefault}{\bfdefault}{n}

\begin{document}

\begin{equation}
    \mathit{TRUC} = \frac{1}{10} \times \sum _{d=0.2}^{12.8} \frac{\pi \times d \times A_{d}}{E_{d}}
\end{equation}

$\mathbf{ABC}$

\end{document}

Otherwise you need to restore the standard math codes for the digits.

\documentclass[]{scrbook}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathtools}

\usepackage[libertine]{newtxmath}
\usepackage{fontspec}
\usepackage{libertine}

\AtBeginDocument{%
  \DeclareMathSymbol{0}{\mathalpha}{operators}{`0}%
  \DeclareMathSymbol{1}{\mathalpha}{operators}{`1}%
  \DeclareMathSymbol{2}{\mathalpha}{operators}{`2}%
  \DeclareMathSymbol{3}{\mathalpha}{operators}{`3}%
  \DeclareMathSymbol{4}{\mathalpha}{operators}{`4}%
  \DeclareMathSymbol{5}{\mathalpha}{operators}{`5}%
  \DeclareMathSymbol{6}{\mathalpha}{operators}{`6}%
  \DeclareMathSymbol{7}{\mathalpha}{operators}{`7}%
  \DeclareMathSymbol{8}{\mathalpha}{operators}{`8}%
  \DeclareMathSymbol{9}{\mathalpha}{operators}{`9}%
}

\begin{document}

\showthe\mathcode`1

\begin{equation}
    \mathit{TRUC} = \frac{1}{10} \times \sum _{d=0.2}^{12.8} \frac{\pi \times d \times A_{d}}{E_{d}}
\end{equation}

$\mathbf{ABC}$

\end{document}