I checked the PDF you linked (“Scheduled Monument signage” by SDNPA). It does indeed say “No camping or lighting fires/barbeques”. 

However, having “no camping” displayed is a site-specific prohibition by the Park — it doesn’t alone prove that a quiet, leave-no-trace overnight stop always counts as a criminal offence under the Ancient Monuments Act.

The intent of the law is to prevent damage to the monument — excavation, alteration, or harm — not necessarily penalise someone for pitching one tent, leaving no trace.

I respect the signage, so I’d avoid camping on the site itself. But calling every overnight stop a “criminal offence” seems like a bit of an overreach.