5% less insolation is still a big deal
542 Mya the insolation would've been 95% of the present day. Bigger than any solar variation we've seen in recent times. Explains why the CO2 was so much higher yet the climate was relatively clement.
For comparison a 2% dip in insolation is sufficient to cause runaway glaciation - a Snowball Earth - according to the climate models used by Michael Hart's evolution models of Earth's atmosphere. His models didn't factor in CO2 build-up when the oceans froze over, so his Snowball events were permanent, but the basic finding remains the same. We're on a knife's edge - if it weren't for the natural CO2 cycles.