My first impression is also in agreement with the parent. The blog post appears to use some terms loosely in order to make the connection between overfitting and Goodhart's law stronger. For example - calling training sample "proxy" and stating that it is is a slightly different goal is already leading towards the pre-defined conclusion.

And the reply also leaves me with a similar impression:

> your training objective can be different at train and test time

But this is not overfitting, this is concept drift, a different and well-defined thing in ML.

> the datapoints you use can be different at train and test time

Both train and test data came from the same population. They are just different incomplete random samples.

I guess what I am getting at - overfitting happens because we know we are training a model on an incomplete representation of the whole. But that representation is not a proxy, as suggested in the article - it is not slightly different to the goal. It's an incomplete piece of the goal.