Answer
The model estimates the population in the year 1000 to be: $493$ people, which was obviously not the case.
(The model is not applicable for periods many years in the past.)
Work Step by Step
The year $1000$ was $950$ years BEFORE $1950$, so we set: $ t=-950.$
$P(-950)=2.56(1.01641)^{-950}\approx 4.928983\times 10^{-7} $ billion
A billion is $10^{9}$ so, after multiplying, it turns out that the model gives an estimate of $493$ people.
Obviously, this is not correct. So, we comment on the model as being inaccurate for many years in the past.
