The point of the OP is that among different implementations for the same interface, choose the simplest one unless you have empirical evidence that compels you to do otherwise.
How does a List map values to key? How does it even represent them?
The problem with OP comment is that KISS and premature optimization are not diametrically opposed. Thet are two separate principles that mean different things.
Premature optimization is bad, but not because it necessarily violates KISS. Similarly, many people overcomplicate code for reasons nothing to do with optimization.
His argument reminds me of people who argue against free speech generally because we already ban people shouting fire in a cinema.
Not efficient, but very much possible.