The main argument sandals vs thongs is about attachment to the foot. Even the most casual workplaces I have been in, require the shoe to be attached to the foot for 2 reasons. 1) In an evacuation, they don't become a hazard 2) For aesthetics, they don't make the slap slap slap noise which draws attention to someones basically bare feet. No idea if there is any studies to support (1).
Also, anecdotally notice most people when they wear thongs drag their feet (myself included), which looks, frankly, Neanderthal. Not exactly the classy look they're wanting to attract to their premium lounges.