Bitcoin Core wallet and Blockchain.com
Did the action of creating a Blockchain.com/Blockchain.info account mean I have handed over my wallet to the company

Yes.

and lost this BTC?

You are wholly dependent on the continuing good-will and competence of the business and its customer support systems and staff.
When Satoshi Nakamoto invented Bitcoin, his famous white-paper said

What is needed is an electronic payment system based on cryptographic proof instead of trust,
allowing any two willing parties to transact directly with each other without the need for a trusted
third party.

I feel too many people ignore this.