You're not really understanding. This guy says they've got forensic methods to determine which ballots are fake and which ones aren't. Basically, the mail-in ballots are folded like thousands of times and each of these fold leaves forensic markers that they can detect through analysis. They can also detect which ballots have been printed to the standards set by the state or the county, whichever one issued the ballots. There are forensic markers they can use to detect which printer a particular ballot came from. They can also determine whether or not a ballot was filled in by a machine or by the same person based on certain features you make when you fill them in.
In summary, they can figure out which ballots are the fake ones. They just need access to the ballots themselves and the digital files in native, non-encrypted format.
Whereas with a signature match, all a signature match will tell you is that ballots were faked but not which ones.