It's interesting! It seems fixable, based on information later on in the article ("Can I get DDR3 DIMMs that are Rowhammer-free?"), and ECC only seems to be part of a solution, and not a complete solution.
But you're still right, and I was still wrong: ECC alone isn't good enough.
Ultimately, nothing that an attacker has physical access to can be completely secured, we can only raise the cost in terms of time and money to attempt to breach the system. Even a system with tamper-destructive enclosures have seen attacks (it's just more expensive and difficult than other attacks).
In short, the more annoying/expensive you can make it to attack your system, the smaller the set of attackers becomes.
That’s why people get mad when you don’t wear a mask even if you “don’t care about getting covid”.