Because language is imprecise and depends on context - you and the GP are talking past each other. They are using the word in a conversational context and you are using it in an academic/technical sense.

From https://en.wikipedia.org/wiki/Pension:

"A pension is a fund into which a sum of money is added during an employee's employment years and from which payments are drawn to support the person's retirement from work in the form of periodic payments. A pension may be a "defined benefit plan", where a fixed sum is paid regularly to a person, or a "defined contribution plan", under which a fixed sum is invested that then becomes available at retirement age.

The common use of the term pension is to describe the payments a person receives upon retirement, usually under pre-determined legal or contractual terms. A recipient of a retirement pension is known as a pensioner or retiree."

A 401k is a savings account, not a contractual obligation, so that's why it's not a pension in common usage.

You're being willfully obtuse here. No one in the US would call their 401k a pension.

Obviously someone who wrote this Wikipedia article would!

Pension is being used as a term of art here not how anyone uses it in standard parlance.