As the return, or "negative" path is entirely wireless on the locos as made at the factory, I cannot see any need for a "negative" return wire at all.
The confusion seems to stem from the supplier using a Scalextric car RX motor in the illustration.
As Scalextric cars are mainly plastic, and the pick up and return are via "braids" that run along the metal strips either side of the slot, then a "negative" return wire IS needed.
The ring terminal is attached to the magnet screw (or bolt) between the motor bottom plate and the magnet screw nut.
The photo is at an angle, so that the nut isn't visible, and it does indeed look like the wire is soldered to the motor itself. But it is not.
The "pin" is the same as used on early Tri-ang Railways and Tri-ang TT Gauge locomotives.
It is a tube that fits over the insulating sleeve on the brush spring. Basically, it's an eyelet, rivet, similar to those used to fix couplings to wagons.
This is instead of the later brush clip, that fits between the brush and the fibre board brush retaining diaphram, as shewn in the service sheet.
The "pin" fitting will work ok. Care needs to be taken that no part of the pin contacts the metal of the motor, or the brush spring, otherwise a short circuit could be caused.
The suppression capacitor would have a ring terminal on one wire. This would be attached under the head of the motor fixing screw, above the fixing lug on the motor bottom plate. As on the service sheet.
Some locos have the suppression capacitor ring terminal attached to the chassis with a separate screw. Generally these are earlier models.
The other wire from the suppression capacitor should be attached to the "positive" feed wire.
Factory wiring from the pick up plate is in two pieces, as on the service sheet, with the capacitor wire soldered into the join between the two pieces. Often there is an insulating sleeve over this three way joint.
The supplier should really think about supplying the correct wiring for locomotives, rather than that suited to a Scalextric car?
This supplier does also do a site for Scalextric cars.