I would guess that Weiss use a lot of motors, so it would not be difficult to get their motor manufacturer to make a "bespoke" motor with a hollow shaft with an internal spline or drive key in place of the standard shaft. Even if it cost a bit more for the special motor, it would probably be more than paid for by savings from the removal of other drive components & assembly time reductions. So while it is "direct drive" to the spindle drive shaft, it is not a "motor spindle unit".
I have only come across "motor spindle units" on aerospace aluminuim routers – these were squirrel cage motors especially built for the task, with heavy duty frames, high precision bearings & ISO 40 taper spindles run from inverters. During the rebuild of one such machine (3 spindle Marwin Max-e-trace) we upgraded the bearings to allow an increased speed (12,000 rpm IIRC – the limit of the bearings) and replace manual drawbars with hydraulic units (new spindles). The ISO 40 tooling was manufactured with the carbide cutters machined integral with the tool shank for maximum rigidity. One of the reasons for the upgrade to hydraulic clamping was to get a positive tool ejection, as the tools got so hot in use that they tended to stick in the spindle – the operators wore welders gloves to change tools. Spindle bearings lasted around 10 months of 24/7 operation.
Several motor manufactures supply "kit motors" for machine tool use – basically a hollow permanant magnet rotor & a cartridge stator unit. The rotor is shrink fitted to the machine spindle & the stator unit is placed in the headstock casting, with the usual high precision bearings in the headstock end caps. Both air & liquid cooled versions are made. Expensive toys !
Nigel B.