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.
Are you saying this is not true?
This is official github page for this secure element:
https://github.com/Infineon/optiga-trust-mThe MIT License
https://github.com/Infineon/optiga-trust-m/blob/develop/LICENSEIt is not related with Trezor, but github clearly shows that Optiga Trust M secure element have MIT open source license code, and that can be confirmed.