Device-independence does not mean "no device," but rather that how the user signs should be the same no matter what device is used for recognition.
The more devices a system is compatible with, the more useful the system. The kinds of devices one would consider for accepting text input include keys and switches, gloves with embedded position sensors, tones injected into one part of the environment and searched for in other parts, and vision systems depending on either visible light or infrared light (useful for security at night).