This point is entirely invalid.
Trezor is running on a closed source microprocessor chip, so this makes it no different than a design with closed secure element chip.
In fact every wallet in existence is running on closed source hardware.
Even if you design your own processor and implement it using FPGA, who says that the FPGA isn’t flawed, this has happened before.
Even if you fabricate your own ASIC, who says that the factory doesn’t insert backdoor circuits, again this has happened before.
Why do you trust STMicroelectronics M3 more than anything else (secure elements)?