If someone shows up with a phone, Open Whisper Systems can't read the messages stored on it if they don't have the passphrase. The messages are encrypted at rest, so they can't create an update that would circumvent it. The only option would be brute-forcing the passphrase. In that way, it's fundamentally different from Apple.
Sure, but the segment that mentioned other encryption apps was talking about what would happen under a US government mandate. Open Whisper Systems would have to comply, move or cease operating (Just like Apple).
Apple also improved the latest iPhones, the OS replace bypass at question here will no longer work. So Signal has an advantage over older iPhones, but not all of them.