This is because they were looking to support a policy position to stop oil usage and trying to build a scientific case, opposed to the other way around.
It was always the opposite of a lie: a logical truism.
If you have a limited resource and an increasing rate of use, there will a point in time where that rate of use peaks, i.e. reaches a maximum and then declines. That's just (very) basic mathematical truth.
And scarcity incentivizing innovations in extraction and processing changes absolutely nothing about it, those things are just (part of) the reason why the peak will be followed by a gradual decline rather than consumption continuing to rise until the day before all reserves are depleted.
Plus, they've always done it before in a similar situation.
King Hubbert's prediction was true.
Campbell and Laherrère's prediction was also true.
(You can say that they failed to predict the tight oil and bituminous sands boom, focusing instead on the (thankfully still completely uneconomical) oil shale potential, but the gist of them being extremely dirty and expensive also fits.)
Climate change was always a lie. As the planet warms, the premium for a safe and healthy planet increases, and innovations in clean energy technology continue progress.