I think that the source of that deviation is from the fact that bidi changes the value of \oddsidemargin. Note that by default marginnote should be touching the right border of the marginpar part. Here is a quick patch that should solve your particular case
%!TEX TS-program = xelatex
\documentclass[oneside,leqno,fleqn]{book}
\usepackage{marginnote}
\usepackage{regexpatch}
\makeatletter
\xpatchcmd{@mn@@@marginnote}{\else\kern\oddsidemargin}{%
\else
@mn@if@RTL{%
\kern\dimexpr\mn@pagewidth-\marginnotetextwidth-\oddsidemargin-2in\relax
}{%
\kern\oddsidemargin
}%
}{}{}
\makeatother
\usepackage{geometry}
\geometry{%
left=70mm,
textwidth=130mm,
marginparsep=8.2mm,
marginparwidth=50mm,
showframe
}
\usepackage{polyglossia}
\setdefaultlanguage{hebrew}
\setotherlanguage{english}
\defaultfontfeatures{Scale = MatchUppercase , Ligatures =TeX}
\newfontfamily\englishfont{Latin Modern Roman}[Script=Latin,Language=English]
\newfontfamily\hebrewfont{David CLM}[Script=Hebrew]
\begin{document}
לורם איפסום דולור סיט אמט.
\marginnote{זו הערה שצריכה להופיע בשולי הדף}
\end{document}

With lualatex you wont get this deviation
%!TEX TS-program = lualatex
\documentclass[oneside,leqno,fleqn]{book}
\usepackage{marginnote}
\usepackage{geometry}
\geometry{%
right=70mm, %<-- note the change
textwidth=130mm,
marginparsep=8.2mm,
marginparwidth=50mm,
showframe
}
\usepackage{polyglossia}
\setdefaultlanguage{hebrew}
\setotherlanguage{english}
\defaultfontfeatures{Scale = MatchUppercase , Ligatures =TeX}
\newfontfamily\englishfont{Latin Modern Roman}[Script=Latin,Language=English]
\newfontfamily\hebrewfont{David CLM}[Script=Hebrew]
\begin{document}
לורם איפסום דולור סיט אמט.
\marginnote{זו הערה שצריכה להופיע בשולי הדף}
\end{document}
\rlapso it has zero width and sticks out on one side, but it's getting the wrong side in rtl contexts by the look of it. – David Carlisle Mar 29 '22 at 13:20\if@RTL, which is not the case here, as the problem happens only withxelatex+bidi. See my answer. – Udi Fogiel Mar 03 '23 at 01:42