Looking at the link from OP, that specific data point seems to be based on the results of this study:

https://www.researchgate.net/publication/325118509_Determination_and_Quantification_of_Household_Solid_Waste_Generation_for_Planning_Suitable_Sustainable_Waste_Management_in_Nigeria

There's a PDF provided there by the researcher. Methods are on page 3, but to paraphrase, it's based on a stratified random sampling of 100 households (a total of 334 people) in Sapele.

A relevant quote from the results:

>By percentage composition, food waste has the highest (75%), the composition of food waste consists mainly of food left-over, vegetables, fish and meat waste, fruits, peels (cassava, yam, potato, orange, pawpaw, banana, plantain etc.).

They didn't really go into why so much waste is being generated, only suggested that much of this could be composted or used to generate green energy.