Every transistor is a phototransistor. You don't even need UV light, just regular amounts of blue light, or large amounts of light anywhere on the color spectrum, can interfere with operation. Badges with COG silicon dies on an OLED or e-ink display regularly stop functioning, when taken outdoors, if the die isn't covered.

The inside of DEF CON event halls are the opposite of bright, so I would expect everything to function fine at the event itself, although outside in Las Vegas in August is about as far from dark as possible on earth, so the badges might not do so well when leaving the venue, depending on how well they are covered. A mostly translucent plastic package can still stop and disburse a lot of light, so they might manage, but I wouldn't rely on it.