There's so many ways to look at timber strengths, and the article lightly covers how their process increases different facets:
> The team’s compressed wood is three times as dense as the untreated substance, Hu says, adding that its resistance to being ripped apart is increased more than 10-fold. It also can become about 50 times more resistant to compression and almost 20 times as stiff. The densified wood is also substantially harder, more scratch-resistant and more impact-resistant
I'm not sure if Specific Strength is something I've ever encountered wrt timber, because it depends on its use whether its compression, elasticity, hardness, or other that determines whether it's good for a specific use. That said, in my country we typically use F numbers, which are (I think) based around a loose series of terms related to stress.
For instance, here's an Oz timber that I know is pretty hard: https://ironwood.com.au/grey-ironbark-technical-specs/
That said "stronger than steel" is what ironbark is renowned for. My brother, a cabinet maker, just made some furniture form the stuff and before the job, his thicknesser and jointer blades were pristine. Afterwards, they're badly chipped. Sparks literally fly when machining this stuff.