Hi Johnny,
I think you have a point. The Buddha said that the Dhamma is something completely unknown to anyone at that time. But people at that time had seen the suffering in the re-birth process (i.e. the hell worlds, animal realms etc.). So the dukkha sachcha cannot simply be the suffering in the re-birth process but something a bit more subtle that no one could fathom. Hence the term sachcha, without simply saying dukkha.
I guess different people will understand it in different ways. Some would see this subtle sachcha better through the re-birth process while others will understand it through the daily life (without having to look at the rebirth process).
Also this dukkha sachcha is more the mental suffering than the pure physical suffering. This mental suffering is something that most will not see. A Buddha doesn’t need come forward to explain about physical suffering. Any human knows what the physical suffering is. So dukkha sachcha is a more subtle, but very important concept.
I think Lal has explained this fact – that there is a difference between dukkha and dukkha sachcha – in detail in many posts as well.