Manual Motor Controller

This most likely depends on some specific technical and regulatory definitions in the governing local electrical code. There probably is not a "one size fits all" answer.

A motor disconnect might theoretically be called on to interrupt any of these various currents:
a. Normal running current
b. Starting current (at various stages of whatever type of starting)
c. Locked rotor current
d. Short circuit current (this is not as likely, usually being handled by other devices)

The local code/regulations may dictate this. From a generic view, (c) may make the most sense.

The Canadian electrical code defines a disconnecting means as a motor disconnect as a device that will safely disconnect the ungrounded conductors in the motor circuit simultaneously.

A motor controller does not have to open the ungrounded conductors of the circuit simultaneously. Not sure why you wouldn't, but then I did not write the code.

Other than that, no difference that I can tell.

Probably a left over from grandpa's time.

"A motor controller does not have to open [all of] the ungrounded conductors of the circuit simultaneously. Not sure why you wouldn't, but then I did not write the code".

To this point: It's not the simultaneously aspect. It is perfectly acceptable to "control" a 3 phase motor, i.e. turn it on and off, by opening only 2 of the 3 phases. It's done all the time on HVAC systems because, I kid you not, a 2 pole contactor is a dollar cheaper than a 3 pole contactor at the OEM level. In single phase motors, you can similarly open only one side of the circuit to turn it off. This concept then can be legally inclusive of "manual" motor starters. So technically, a manual motor starter does not have to open ALL of the ungrounded conductors to function simply (only) as a motor controller.

So you can't make a general statement that a "manual motor starter" by no other definition will be considered a "Disconnecting Means" in and of itself. Practically speaking, 99.9% of 3 phase manual motor starters are 3 pole and all 3 poles open together, so the distinction is relatively meaningless. But on 1 phase motors, it's very common for a MMS to be only a 1 pole device even when using 2 ungrounded conductors, and as such will NOT be a suitable disconnecting means from a safety standpoint. So including that "suitable as motor disconnect" distinction in the description is a good CYA move.