Yes, you can use SX OS in handheld mode, but there isn't much of a point in doing so for the vast majority of people.
While it's true USB devices can be used in handheld mode on SX OS, it isn't the most portable system if you're using a USB HDD storage device. If you're using something more portable like a thumb drive, it has to be USB-C, and it isn't going to have the same benefits as the aforementioned HDD (large and cheap storage space). In other words, portable USB sticks tend to come in about the same sizes and prices as SD cards.