Are you sure they're not just driving a manual transmission car? I can keep my foot lightly on the brake and apply the clutch to accelerate, without touching the accelerator pedal (diesel engine has a lot of low-end torque). Not that I do this in practice, except in drive-through lines just for the fun of it.

In first gear I can reach 8 km/h, but it's possible to get all the way to fifth gear while idling (takes a long time though).

Perhaps, but left-foot braking is a useful technique, and common practice in some forms of racing.

In street driving, the main application is being ready to brake while driving normally. It is much faster to brake with the left foot hovering over the brake pedal than by moving the right foot from the throttle to the brake. Of course covering the brake pedal with the right foot precludes driving normally on a flat road or uphill.

This should of course be practiced in low-risk situations before being attempted in high-risk situations.