Remember that, theoretically speaking, a USB transmitter just takes a signal, shifts it up in frequency, and amplifies it. That means that if you take a 50Hz wide 8FSK signal centered at 1500 Hz, and you feed it into a USB transmitter with a dial frequency of 14.074 MHz with all of its voice processing and such turned off, what you get is a 50Hz wide 8FSK signal centered at 14,075,500Hz. And that's all that's happening here.
FT8 is FSK on the air, and it's FSK between the computer and the radio. Just one is at radio frequencies, and one is at audio frequencies, and a radio set to "USB" turns one into the other. A radio set to "LSB" would also (almost) work, except that all of the FSK shifts would come out backwards, which prevents it from decoding unless everyone else also uses LSB.
HF packet works exactly the same way — a TNC or PC generates a 300-baud FSK signal at a frequency pair of, say, 1600 and 1800 Hz, which is then modulated up to (e.g.) 10,149,200 and 10,149,400 Hz by a radio in USB mode with a dial frequency of 10.1478 MHz.
Some small dedicated transmitters actually do FT8 by directly adjusting the frequency of an RF synthesizer, which is "classical" FSK — although it's more commonly done for WSPR or FSK-CW than FT8.
The QRP Labs QDX is a fun case because it also transmits FSK modes by adjusting a frequency synthesizer, except it decides what frequency to use by "listening" to the audio coming into its sound card and picking out the dominant frequency. So it's a radio that interfaces to a computer in the same way as a USB transmitter, but it's only suitable for FSK modes because it can only output a single frequency at once, at a fixed amplitude.