It was thinking that 15 is the most common answer on the Internet to the request "sum the numbers above".
The COPILOT() feature does not have access to your spreadsheet. You pass it the cell range it is supposed to use for context in the optional second parameter. The user did not provide any second parameter, so they're just asking it to generate some text based on the text of the prompt and nothing else.
The person who made this tweet either didn't bother reading how to use the feature, or is being disingenuous.
It couldn't even see the spreadsheet.
COPILOT()can't see the spreadsheet, only the range you provide in the second parameter. None was provided.