They print the filament, agreed, but because they are using a machine with a print bed they are limited in how much they can print. Imagine you had what was essentially a 1 axis 3D printer. That printer had what would look kind of like a tank tread with a bunch of very small (~10mm^2) steel plates that were hinged like a tank tread, and formed a loop along the printers single axis. Say there was roughly 40 cm or tread that would be level. The printer, then prints a few cm or filament along the plate and the set of plates move underneath the print head. As they get far enough away, the plates drop down along the idler wheel and the printed filament attached to that plate is popped off and hanging in air because it is still attached to the rest of the filament that has been printed.

If, of the end of that tread/surface you had a take up spool you could just print until you filled up the spool if you wanted too.

I believe a 1 axis printer is usually called an extruder.