Yes, Trezor 3 Safe is using open source secure element model Infineon OPTIGA Trust M, and that can be verified.
Most of the other brands are using secure elements that are closed source.
What makes you say that? There is literally nothing open source about this chip, nor any secure element, nor
any MCU or processor used in any hardware wallet or computing device.
To my understanding, Trezor just made some claims about being able to publicly disclose vulnerabilities because they didn't sign an NDA with the manufacturer.