NO!
In as much as it seed phrase can be imported into another wallet in order to access the private keys and blockchain, it should be called a non-custodial wallet. The reasons that some wallets are closed source isn't arrogance and in some cases these wallets started as open source and later turned to closed source.
You are wrong.
Just because it provides a seed phrase does not mean it is non-custodial. Since the wallet is closed source, you have absolutely no idea how that seed phrase was generated, how it is stored, who else has access to it, and so on. For all you know, Coinbase (or Coinomi, or Trust wallet) have a list of every seed phrase their wallets have ever generated stored on a server somewhere.
I'd also point out that the reasons Trust wallet gave for moving to closed source are bullshit:
I know this is the reasoning Trust wallet give for being closed source, but I don't buy this reasoning at all. The only part of a wallet which 99.9% of users pay attention is the GUI. It is trivial to clone a GUI even without access to the source code. Being closed source might keep all the back end, the wallet generation process, the signing transaction processes, etc., hidden from attackers, but attackers do not care about any of that in the slightest. All they need is a wallet which looks the same as Trust wallet, which sends any generated or entered seed phrases to their server online. So they can use any bare bones code which generates seed phrases, add in their malicious code to send those seed phrases to a server, copy the GUI just by looking at it, and release it to the app store as "Trust Wallet". Being closed source does nothing to protect against this.