Why lemon juice is thought of as alkalizing despite its acidic pH?
Source: https://www.healthline.com/nutrition/lemon-juice-acidic-or-alkaline#TOC_TITLE_HDR_4

A food’s pH does not determine whether it will create an acidic or alkalizing effect once digested. It is the acidic or alkaline byproducts that are created once the food is digested and processed in your body that determines the acidic or alkalizing effect.

This effect is measured by the Potential Renal Acid Load (PRAL) which takes into account how much acid your kidneys need to filter out. Your kidneys try to maintain a particular blood pH balance by extracting excess acids or alkali into the liver and out through your urine. Proteins, such as grains and meats produce more acid (positive PRAL) while fruits and vegetables produce more alkali (negative PRAL).

Lemon juice, once metabolized, produces an alkaline byproduct and so it has a negative PRAL. This is why lemon juice is said to alkalize the body.