In the first place, no one said they needed to, only that they probably did.
Secondly, it's not "3000 extensions". They didn't somehow magically divine that the 2953 (+/-47) extensions we see here were the ones that they needed to download in order to be able to exploit the content-accessible resources described in their extension manifest. They looked at a much larger set, and it got filtered down to these 2953 that satisfied the necessary criteria.
