The Reddit question he responded to stemmed from an alternate version of the video, which was directed by Jason Matzner and featured the band performing in flooding basement. A clip of this video, which you can watch below, surfaced online in 2011.
Check out the explanation from Hoppus, below.
"i had an idea for the josie video, that we would be the band playing on the deck of an old cruise liner as it sunk. we would play in real time as everything around us exploded in slow motion. gradually at first, building in intensity, reaching complete catastrpohe at the end. people running, sparks flying, superstructure collapsing. the ship floods, and the last shot of the video is the ship sinking into the dark waters as the song ends "everything's gonna be fine." the label listened and said "cool, but no. that video will cost millions of dollars." so the alternative was us playing in a basement and the pipes break. the room floods and we continue playing. we shot the video in the backlot of universal studios on a very cold and long day. i just remember shivering and the water always being shockingly cold every time we had to get in it. bracing. so cold. the first edit came back and no one was really feeling it, so we scrapped it and started over."
Other questions Mark addressed included topics such as musical gear, coffee, video games, and what music has recently been on his radar. Check out the full AMA here, and watch the official video for "Josie," below.