Thanks for the clarification. I watched the GenXGrownup review, and there he showed you could save BASIC programs, but that there was no way to access them outside of the machine. It sounded as if The400 makes a private disk image that you can't access from a PC. The review also didn't cover if you could boot from a language cart like ASM/Editor and also have Atari DOS available, like it does for the built-in BASIC.
I'd love to see some more experiments like this, once more units get in people's hands. The ability to have a programming environment on or near-par to original hardware, but with a) smaller desk footprint, b) modern connections for power and HDMI video, and c) modern, high-quality keyboards would make The400 a compelling alternative for hobby hacking directly on the device.