Very good question!
I don't know. But if I am allowed to guess, it could be precisely that. Because it wasn't man made, and man made CO2 was not accumulating, then over time CO2 and warming was absorbed and balanced out. But IF you acknowledge that CO2 worsens warming, then at least we know continuing to produce CO2 won't help, if we assume warming is a problem. That may just be the reason why people are calling for CO2 reduction, because IF the past temperature rises can be attributed to CO2, and not man made, then we know warming will not perpetuate IF we reduce CO2 to the point where it's "manageable".
None of these points will makes sense though, if you believe
1. Globe is not warming
2. CO2 causes nothing
3. Warming isn't bad, or we don't expect destabilization of climate
4. We can't reduce it enough
5. Well, we can, but my freedom is more important
6. Well, maybe it's not, but I still like my freedom.
So I think the argument basically comes down to this.
1. Does CO2 really cause or worse global warming and climate instability?
2. If it does, should we at least prepare for it, or seek to avoid it?