Correct!

Plus, most law enforcement seizes the device and keeps it until after the trial. If there were valid arguments against the authenticity of data in the extraction report, it would be easy to verify that data's existence by checking the phone, re-extracting the data using a known-clean UFED, etc. This isn't the end of the world by any means for legal mobile device searches.

Except that if the device is compromised, it could have changed the data on the phone. The phone, as evidence, can't be trusted anymore.

Signal never indicated this in the blog. They said that the phone would have a file that could be used to victimize the UFED PC after the extraction completes. It's plausible that the UFED could be infected post-extraction with malware that re-establishes a connection to the phone to infect it in reverse, but this is extremely unlikely and it would be easy to determine (assuming the UFED still exists and hasn't been meaningfully modified since the extraction.

For the UFED Touch, for example, the device runs the extraction and saved the result to a flash drive or external drive. This is then reviewed with the UFED software on another machine (laptop, desktop, whatever). What you're describing would mean that the extraction takes place (one-way. The Touch requests an ABD or iTunes backup process, phone responds with the backup files). The the malicious file in the backup pops and executes a command that runs software on the phone, thus infecting the phone with false data to cover the tracks and make the data on the report match the device. This is unreasonably complex, and I doubt any judge would accept it as enough to consider the data inadmissible. Let alone the fact that the data likely exists elsewhere in a verifiable way (Facebook Messenger, WhatsApp, Google Drive, etc), which the initial extraction results should give probable cause to the cops to obtain.