Makes sense because it's quite impossible to use those numerals directly. Fibonacci and the modern decimal numeral system came much, much later.
The emphasis is on "I believe". I would expect some documents which attest that but maybe no such luck. Like "posca", maybe it was considered too common sense to be worth written down.
On the other hand what amazes me is the thought that the entire WW2 was fought flying planes that were designed and built without the slightest trace of electronic calculator coming in aid. There's integrals and derivatives and transcendental functions, how the heck did they manage to do that with pen and paper?
I know they had tables of logarithms and trigonometric functions and some mechanical calculators but come on.