How is it inaccurate? If I only care about buying apples, and apples get 10% more expensive, and my salary only increases by 5%, then I can't buy as many apples as I could have before. How many apples I do actually buy in the end is irrelevant to the calculation.

The person you're replying to erroneously interpreted "stay even" as "avoid going into debt," instead of your income's purchasing power remaining constant.

Consumer price index is about consumer goods. This is why tarrifs and such are considered regressive - they hit people harder the less money they have because a larger percentage of their spending is consumer goods.

If I invest half my income and spend half my income, and the prices of goods goes up 4.2% and my income goes up 4.2%, then I've made progress; I'm now investing more than half my income, because the half of my income I was spending has stayed even and the half I was investing has increased.