I've been following the Amazon story. Don't work for them, but work in a similar sized massive facility, so I can relate.
It seems their 'severe weather shelter area' isn't a shelter at all and just a open spot in middle of warehouse (nothing protecting them from flying debris). Don't know if it would have mattered, as allegedly the tornado hit right at the meeting place, and not everyone was there as there was no warning at all and people had to hide wherever they could find. And Amazon doesn't have intercom or alarm system but depends on a guy walking through with a bullhorn to tell people to seek shelter (in a building large enough to fit several football fields, even with bullhorn you can't hear someone on other side of building). At least one dead actually was inside a bathroom.
My work is better prepared than this, but again, if there really was no warning, it might not have mattered.
Tornado warnings come with almost every storm. Can't close business for every storm. Average lead warning time for tornado is 13 minutes, so often it is far less than this, so no warning at all is possible. Phone alerts go bezerk when a tornado has actually been spotted, but often these are an hour away, so people tend to ignore them. If you happened to be right where the tornado first touched down, obviously tornado has only then been spotted, and no warning.
If tornado already been spotted in area, worst thing to do is get in car, much more vulnerable there.