I agree, but if a wallet is not open source it cannot be non-custodial.
Discussion
Yes it can. For example there's many wallets that haven't released their source code but still allows users to create and hodl their own keys and self custody.
Of course I would not recommend trusting any of those wallets.
Technically thats true. In my mind if I have to trust a software to not send the seed to someone I see it as custodial.