If the use a Subversion/Git code repo to maintain their codebase, they should be able to track down who wrote the code and when.
Or if by "unauthorised" they mean "via unauthorised use of an authorised account" - i.e. one of their dev team had their account hacked.
Even when could be difficult to be confident about, never mind who, especially if the even happened quite some time ago so the amount of other information available for forensic analysis my be minimal (network logs have probably been archived off, maybe to /dev/null, by now).
export GIT_AUTHOR_EMAIL="potus@whitehouse.gov"
GIT_COMMITTER_DATE="Tue Dec 8 12:33:03 2015 +0000" git commit --date="Tue Dec 8 12:33:03 2015 +0000"
That will change the commit and author dates.
Maybe they don't, but this seems like something they ought to do.
A PGP-signed commit with a key generated on a smartcard (and never exposed) is a little better, but ... Someone pwned RSA before, and I'd be surprised if Gemalto and Yubico (just two examples) don't have some Three-Letter-Agency backdoor (and .. I'm sure those TLAs have equipment that can read modern smartcards).