The document referenced in the blog does not say anything about the single transistor multiply.
However, [1] provides the following description: "Taalas’ density is also helped by an innovation which stores a 4-bit model parameter and does multiplication on a single transistor, Bajic said (he declined to give further details but confirmed that compute is still fully digital)."
[1] https://www.eetimes.com/taalas-specializes-to-extremes-for-e...
That's much more informative, I think my original comment is quite off the mark then.
It'll be different gates on the transistor for the different bits, and you power only one set depending on which bit of the result you wish to calculate.
Some would call it a multi-gate transistor, whilst others would call it multiple transistors in a row...