I said specifically it's not a rounding error. You would, at minimum, round to the nearest hundred million years, which was done.

No. First, if the best known number is 66 mya, why throw away almost all the precision and call it 100 mya?

Second, the number it was being compared to in roenxi's comment was 9 million years, not 4 billion years.

So, yes, it is in fact a rounding error.

;-)

As numbers get bigger the amount of rounding gets more acceptable. 100 to 101 is as 1e100 to 1e101, despite te latter technically being off by 90%.

I think 100mya is perfectly fine. It doesn't undermine the point and this isn't trivia I plan to remember more accurately than that.