I worked around it, instead of simpleslides just use larger fonts to with
regular pages

\startbuffer[slides]
\setuptolerance[
  horizontal,
  tolerant,
  stretch]

\setuppapersize[
  A4,landscape]

\setuplayout[
  width=middle,
  backspace=35mm,
  cutspace=35mm,
  height=middle,
  topspace=5mm,
  bottomspace=20mm]

\setuppagenumbering[
  location={footer,center}]

\setuphead
  [section]
  [page={yes}]

\usetypescript[pagella]
\setupbodyfont[pagella,18pt]
\setupbodyfontenvironment[
  28pt][  % For body
  d=36pt, % For title
  a=12pt, % For author & date
  ]
\setuptype[style=medium]
\setuptyping[typing][bodyfont=16pt]

\setupwhitespace[medium]

\setbreakpoints[compound]

\setuphead[chapter][style=\bfd]
\setuphead[section][style=\bfc]
\setuphead[subsection][style=\bfb]
\setuphead[subsubsection][style=\bf]

\setupitemize[autointro] % prevent orphan list intro
\setupitemize[indentnext=no]

\starttext

\startsection[title={MySlideTitle},ref={myref1}]

Body text

\startitemize
\item Item 1
\item Item 2
\stopitemize

\stopsection

\startsection[title={MySlideTitle},ref={myref2}]

Body text

\stopsection

\stoptext\starttext
text

\stoptext
\stopbuffer

  \savebuffer[list=slides, file=\jobname_slides.tex, prefix=no]

  \starttext
  \startbodymatter
  \dorecurse{25}{\input zapf\par}
  \typesetfile[\jobname_slides.tex][--purgeall][object=no, width=0pt]
  \stopbodymatter
  \startappendices
  \getfiguredimensions[\jobname_slides.pdf]
  \dorecurse{\noffigurepages}
      {\startTEXpage
        \externalfigure[\jobname_slides.pdf][page=\recurselevel]
       \stopTEXpage}
  \stopappendices
  \stoptext

Op za 31 jul. 2021 om 16:22 schreef Thomas A. Schmitz via ntg-context <
ntg-context@ntg.nl>:

>
> On 7/31/21 4:06 PM, Pablo Rodriguez via ntg-context wrote:
> > BTW, I cannot get simpleslides working with LMTX.
>
>
> I had to make some slight adjustments but have been to lazy to upload a
> new and improved version. One reason being that I can't remember my
> username and password for the modules section of the garden... I'll have
> to ask Taco to reset it for me.
>
> Thomas
>
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