Answer this: Can you remove content from the blockchain?
If no then the statement is true, regardless of the fact that content can be removed by a consensus of witnesses. Therefore the blockchain is immutable, meaning that it would require impossible circumstances for you to remove content from it.
This isn't some sort of quantum super state.
Nor is it simply abstraction, but real world application, so even if the witnesses can come to a consensus and remove content, nothing short of that will remove the content, and even then there could still be other witnesses that hold a copy of the database.
"Can you remove content from the blockchain?"
Without complete knowledge of the inner workings I will hazard saying yes from what I understand. I believe that a consensus of the top 20 witnesses could push through an altered blockchain but I could be wrong.
Let's assume they can for argument's sake. That would mean that the blockchain isn't immutable since it can be changed in that scenario. This relies on the blockchain being defined as the current live consensus version.
You've added a restriction that it must be changeable by an individual. No such restriction should be expected when calling something immutable. Immutable as a term on it's own in binary, it is changeable or not. I also specifically stated that it relies on the definition of the blockchain being the currently accepted consensus. That does not include backups and rogue chains.
In the fully possible scenario I described it is clearly not immutable. Not everything is a paradox and our universe has binaries.
The probability of it happening has no bearing on whether it is immutable. The ability to change it is baked into it's design. Just like ethereum is not immutable they changed the blockchain.