Well, for one thing, chances are that few, if any, of those mechanics were pilots. Mechanics fix things, but may not be prepared to address what will or will not happen in an aircraft emergency, or how to deal with it. What will occur with a throttle plate largely depends on the system, how the failure occurs, where it occurs, etc.
Marvel Schebler carbs do have spring assist, but weather or not the carburetor goes wide open on the throttle plate depends on a number of factors. To suggest that the engine will go to full throttle during a throttle control failue is fanciful at best, and borderline criminal when informing an inexperienced pilot of what to expect.
An aircraft equipped with an ellison throttle body, such as in certain injection systems, will generally experience a closed throttle system when a control failure occurs. The engine suction tends to pull the throttle plate closed, with the force to do so considerably stronger at lower throttle settings (plate tending toward closed, lower power setting). In fact to quote the Ellison service manual:
2-5 Throttle Linkage
During engine operation at less than full throttle, a substantial pressure difference exists between the two ends of the throttle slide. This pressure gradient causes a strong buoyancy force acting to close the throttle. This force is greatest at idle and diminishes at increased throttle openings.
CautionIn the event of any throttle linkage failure allowing unrestrained throttle movement, the engine will immediately and without hesitation, return to idle.
Because of the higher throttle friction associated with the TBI, linkage installations utilizing a pull cable in only one direction with spring return in the opposite direction are not satisfactory.
Other systems vary specifically within the aircraft. In a given experimental airplane, for example, a rotax 582 engine may fail closed, while the installation on a rotax 912 may fail open.
In the aforementioned marvel schebler installation, the spring assembly will generally not give full throttle, even with a completely disconnected and free throttle arm. If the arm is attached to anything (eg, the break in the throttle assembly is at the control in the cockpit, chances are that it's not moving at all.
The bottom line is that while it's good to dispense accurate advice, don't get in the habit of painting a rosier picture than reality might permit. If a pilot experiences a power change that does not correspond with power lever positoning, or which can't be controlled, or that does not follow the action of the thrust lever, it's time to get on the ground. Yes, the engine might fail to full power, but i might not. Don't bank on it going to full throttle.
In dual-cable installations where one cable pulls the throttle plate open and the other closed, failure of one cable means that the throttle can be moved in the opposite direction, but not back again. That means that if the open-cable breaks, it may remain there, but moving it to closed may mean a permenant change. In a push-pull type arrangement utilizing a wire, failure may often result in the throttle plate staying right where it is when the failure occurs.