Originally had project/build on my account but it was decided to move the build to another account.
Deleted the project/build but the DNS Zone persisted and will not let me add the custom domain housky.io
You still have the DNS Zone hosted and active in the account called “knkjett”. Nothing has been deleted there, so indeed, you cannot use the name in a different account. Since both accounts are yours, you should be able to transfer the zone between accounts:
…and then you can use those hostnames in the new account.