You are right, but your premise is probably different. They were indeed wrong. They were too conservative.
Some historical perspective:
1990: 1st Assessment
Little observational evidence for a detectable human influence on climate
1995: 2nd Assessment
The balance of evidence suggests a discernible human influence on global climate
2000: 3rd Assessment
Most of the observed warming over the last fifty years is likely* to have been due to the increase in greenhouse gas concentrations
* > 66% chance
2007: 4th Assessment
Most of the observed increase in global average temeratures since the mid-20th century is very likely* to to the observed increase in anthopogenic greenhouse concentrations
* > 90% chance
2016
It is extremely likely** that human influence has been the dominant cause of the observed warming since the mid-20th century.
** > 95% chance
2021:
It is indisputable that human activities are causing climate change, making extreme climate events, including heat waves, heavy rainfall, and droughts, more frequent and severe.
Some historical perspective:
1990: 1st Assessment
Little observational evidence for a detectable human influence on climate
1995: 2nd Assessment
The balance of evidence suggests a discernible human influence on global climate
2000: 3rd Assessment
Most of the observed warming over the last fifty years is likely* to have been due to the increase in greenhouse gas concentrations
* > 66% chance
2007: 4th Assessment
Most of the observed increase in global average temeratures since the mid-20th century is very likely* to to the observed increase in anthopogenic greenhouse concentrations
* > 90% chance
2016
It is extremely likely** that human influence has been the dominant cause of the observed warming since the mid-20th century.
** > 95% chance
2021: It is indisputable that human activities are causing climate change, making extreme climate events, including heat waves, heavy rainfall, and droughts, more frequent and severe.