Open source is not the same as non custodial. For example there's custodial elements in many open source bitcoin wallets.
Discussion
I agree, but if a wallet is not open source it cannot be non-custodial.
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.