Flash flooding is not all that rare in Las Vegas–I think people just think it is because of the desert climate. Having grown up in Vegas, it's something that was ingrained in me over the years. This part of summer is monsoon season (which starts up around mid-June and can last until September) where weather patterns can bring on thunderstorms quickly. Due to the type of soil around the valley (most of which is not very permeable) and all the development, quick downpours don't have a chance to soak into the soil and can cause flooding because the relatively high amount of rain in a short time frame can have nowhere to go. Flash flooding has probably occurred less in recent years due to ongoing drought, so it's probably not really at the forefront of people's minds.
The effects of flash flooding in Las Vegas have greatly diminished in the last decades due largely to the efforts of the
Clark County Regional Flood Control District. The CCRFCD has built over 200 miles of flood control channels and 82 flood detention basins in the Las Vegas Valley since the organization was formed in the mid-1980s, and they still have a lot of work to do on their master plan. But even with those efforts, there are still perennial trouble spots. The video in the second tweet showing the flooded parking garage I assume is at the Linq–this is adjacent to/on top of the Flamingo Wash, and I can recall many reports of this flooding during heavy rains when I was growing up (back when the property was the Imperial Palace). Another one was the Charleston Blvd underpass at the UPRR, which would often flood even during light rains (and invariably at least one vehicle would get stuck down there every time) until a project in the last decade finally fixed it. Local news stations would often send reporters and camera to these two locations in advance when strong rains were predicted.
This time of year, it is common to see ads/public service announcements from the CCRFCD reminding people to stay out of flood channels and to not drive through flooded streets–a refrain used for as long as I can remember is "turn around, don't drown". A long-running campaign played off of Nevada's culture of personalized license plates to warn of the dangers or lament boneheaded decisions regarding flooding–this was very popular on billboards around town, and had a few TV commercials over the years (here's a couple from 2010 and 2012 via their YouTube channel).