https://ferrofluid.ferrotec.com/products/ferrofluid-audio/audioselect/
Spider removal: Voice Coil Centering With Ferrofluid
The presence of ferrofluid exerts a uniform radial centering force upon the voice coil in the air gap. The magnitude of this force is dependent upon the strength of the permanent magnetic field and saturation magnetization of the ferrofluid. The greater these values, the stronger the centering force. Reduced scrap rates on the production line, reduced field returns and reduced distortion (due to the suppression of radial and rocking modes of the voice coil) are some of the well-known benefits of this centering force.
In recent years, the availability of high saturation magnetization ferrofluids have allowed designers to completely remove the spider, relying on the ferrofluid to center the voice coil in the air gap. This technique not only yields lower cost and a simplified production process; it also removes the well-documented nonlinearities present in all spiders, resulting in lower distortion. In general terms, the saturation magnetization values of ferrofluids used in spiderless designs should not be lower than 33 mT.
I now if the tech used ferrofluid to center the voice coil because of the difficulty of doing so with the Model F driver. Then, because the wrong fluid was used, it got gooey and froze. So, perhaps the incompetence theory is most accurate.