It's a valid method that avoids some edge cases where Excel's autocomplete gets confused. He just forgot the last step of copy and then paste back into the same cells as text to remove the formula.

My pet peeve is my coworker who finds it easier to write macros to analyze the data instead of setting up formulas, and then I am completely unable to follow his analysis logic because it's all just columns of numbers with no way to tell what was used to calculate what.

We had, let's say very educated people, who would consistently create tables in Excel by hitting Ctrl-A to highlight all the cells and then create the table.

Then they would call me because their sheet kept locking up their computer every time they entered a number, because it's was updating literally all of the possible cells you could fit in an Excel file.

In the end, it was cheaper to pay me 14 bucks an hour to fix it than it was to train this department on how to not do that.