This isn't because of open source code, it's because the Trezor doesn't have a secure element
opensource code hardware wallets are easier to hack
for criminals
because they can search the code for vulnerabilities
every code has vulnerabilities
https://www.coolwallet.io/kraken-hacks-trezor-in-15-minutes/
Discussion
One is to use a tougher coding language that’s more resistant to the above attacks. Trezor’s micropython, for example, is considered to be among the most robust languages on the market. However, this isn't open source, its proprietary and expensive