It's a bit different: code below is ok, and TeX comment is seen "before" lua comment .
Only Hello! is print on pdf

%%%%%%%%%%%%%%%%%%
 \def\Foo{%
\startluacode
-- this is a lua comment
%%  this is a TeX comment
\stopluacode
}

\starttext
Hello !
\Foo
\stoptext
%%%%%%%%%%%%%%%%%%%




This is wrong

%%%%%%%%%%%%%%%%%%
\starttext
Hello !
\startluacode
-- this is a lua comment
%%  this is a TeX comment
\stopluacode
\stoptext
%%%%%%%%%%%%%%%%%%%



! LuaTeX error <main ctx instance>:3: unexpected symbol near '%'.
\@@expanded ... comment
%% this is a TeX comment
}
                                                 
l.6 \stopluacode
               
?