I can't speak for this individual, but when kids are involved, it's normal to get tested and confirm, yes, you've gotten infected with influenza.

A year pre-covid my whole family (kids first) got the flu. Viral test confirmed one kid got two strains in succession ("A" and "B").

Its sudden onset, sudden remission, total body exhaustion, lack of my traditional cold symptoms, etc. made it very obviously different from any cold I'd had. Previous to that, I thought "the flu" was some strong "the common cold". Or just a word for arbitrary viral infection (e.g. "stomach flu").

I understand "the flu" has an ambiguous meaning now in English, but I'll certainly recognize that kind of infection again.