Um no.
I show you this to prove to you that 1 eth confirmation isn't equal to 1 bitcoin confirmation. If it was anywhere equal in security, there would not be a >10X difference. And also multiplying confirmations by average block time and seeing that it is lower than bitcoin doesn't mean coinbase thinks it is more secure. It can also mean that the value of the average eth transaction on Coinbase is lower.
A 51% attacker will be able to churn out 100x the number of confirmations if confirmations were made 100x smaller. It doesn't matter how fast you make them or how many tricks like GHOST you add.
These are some helpful websites/articles on understanding confirmation security.
https://medium.com/@nic__carter/its-the-settlement-assurance...