It is not known that Cellebrite helped them. The rumors that they did stem from a contract they have with the FBI that is unrelated.
In fact the contract they have with the FBI doesn't supply anything that can break into an iOS 9 device so unless they had a separate product line not included in a contract only two months old it's unlikely it was them.
Cellebrite has allot of products that aren't advertised directly which are provided through their CAIS services (and there are also at least additional 2 unlisted service tiers).
They have allot of turnkey solutions for various markets as mobile forensics is only a part of their portfolio, but quite a few of their forensic services and products are not publicly listed.
*I worked for an Israeli information security firm that is a research partner and service provider for cellebrite.
In fact the contract they have with the FBI doesn't supply anything that can break into an iOS 9 device so unless they had a separate product line not included in a contract only two months old it's unlikely it was them.