I don't think anything is brake by wire, even that Lexus has a failsafe mode that ultimately is a direct physical connection. No saving parts there yet.
Like the other commentor said, the brake is still connected to the brake pedal and the car mechanically assists for one-pedal mode. You can always still use the brake pedal even with any automation or assistance off.
<thing>-by-wire means there is a control with no physical connection to <thing>.
for example, older tesla cars can steer themselves, but the steering wheel still connects to the front wheels mechanically.
The newer cybertruck has a steer-by-wire. The wheels are turned side to side by motors, but the steering wheel is not mechanically hooked to it, it's just an electronic controller.
The one-pedal though is only a mechanical assistance to the existing connection between the brake pedal and the brake. Maybe we're splitting hairs, but its more akin to power steering.
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, I think; 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 that one-pedal-mode provides.)
(If that sounds aggressive, then I apologize. I'm still recovering from a vacation with a 30-hour drive, and I may be up too late, and may also be drinking a bit much tonight.)
