Well, biological neurons are much more complex than CS neurons (https://www.quantamagazine.org/how-computationally-complex-i...).

Also, you're working under the assumption that they are equivalent between mammals which as far as we can tell it's not the case (https://www.medicalnewstoday.com/articles/why-are-human-neur...).

So my guess is that the comparison is much more complex than just number of neurons.