Why can't this mean 'Is it ready to start'?
It does now ;) but keep in mind a sentence like this would generally refer to a female
Why not 'he' is ready to start?
Because it says pronta, which indicates a female. It would be pronto for a male.
Shouldn't it be 'Ė pronta ad iniziare'?
It might be, but a and i are not the same vowel sounds so it's not mandatory.
why is it she not he?
As was already answered 4 years ago in the comments that you didn't bother reading. If it was he is ready it would be è pronto, but it's è pronta so it must be she is ready because it has to agree with the implied pronoun.