With [traditional] power-assisted steering, there is always a direct mechanical connection between the wheel and the steering box/rack. It's assisted, usually hydraulically, but it is not isolated by a wire; that direct mechanical connection is always there.

With [traditional] power-assisted brakes, there is always a direct connection between the pedal doing the braking and the wheel cylinders. It's assisted (usually, but not always, by vacuum), but that direct mechanical connection is always there.

With one-pedal driving that only uses exactly one pedal, there is never a direct mechanical connection between that singular pedal and the braking system. That connection is only electrical; there is no direct mechanical (or hydraulic, to split hairs) connection at all. It is entirely brake-by-wire, unless one chooses to place a foot on the brake pedal and thereby do something other than one-pedal driving.

Thus: Unless there's hydraulics connected to the accelerator pedal, then: As long as mechanical braking (pads-on-rotors friction) can be performed in one-pedal mode (and it can be), then one-pedal mode must perform braking-by-wire.

We're already there; people use brake-by-wire every day. (That we also have a functional hydraulic brake pedal as a backup does not mean that brake-by-wire is not a thing in one-pedal mode.)