It's not completely out of the norm for Comic Con. Metric did a secret show after a Scott Pilgrim screening there back in 2010, Childish Gambino did a show there in 2011. This would definitely be one of the bigger names they've had. Also to kinda clarify, they won't be performing at the convention itself, but in the area near it. SDCC takes over most of the city surrounding the convention center so there's always tons to do. There's a House Of Blues not far, but it's seemingly all booked up, except for the 27th, and the show on the 25th is a late show, so it's still feasible that LP could play there by themselves at like 8, depending on if HOB would wanna clear out/clean/set up for the next show by 11.