It’s a reaction to a reporter at a NASCAR event hearing the crowd yell “Fuck Joe Biden” and pretending they said “Let’s Go Brandon” - they basically just ran with it. The entire connection between the two is a reporter openly lying about what a crowd was audibly yelling. This resonates hard with the sort of people who believe the mainstream media (meaning all major news media except the largest cable news network, of course) is extremely deceitful at every turn to protect a Democrat agenda.