Hello nice people!
Such theories abound and none uses to be confirmed when checking more samples. Among competing theories, one claims that wood improves over the centuries as it loses mass. Others claimed the wood was stored in ponds for decades. Or that the climate was colder then (but just an altitude 120m higher does the same).
As a general warning: mistrust all such claims, especially the spectacular ones. Be they correct or not, they find too easily a path into the general Press to bring career advantages to the authors, especially since the Shanghai rating.
For instance "Guarnieri, Guadagnini and Stradivari aren't better" is just cr*p. Violinist do recognize old Italian violins, or rather good instruments. The difference is bigger than for wind instruments. Tests claiming "no difference" are just badly conducted.
You notice differences better when playing the instrument than when listening at it. Some affect the ease of playing, not the sound.
Musicians hear better than an average journalist.
It takes time to notice the differences. I saw a "test" that ran a scale up and down, end.
One must target artificially, provoke, check and reproduce the differences. Once you noticed them, you hear them in normal music too. Playing only some Mendelssohn won't tell anything in 5min. But you would notice the differences after days of normal use.
Yes, some less old instruments are excellent, equalling arguably old Italian ones. More and more soloists (Hilary Hahn, Vilde Frang...) play instruments made by Vuillaume.
One should also remember that the old Italian instruments differ much now from the original sound.
The string materials have changed. Huge difference, stronger than between two instruments.
The bridge's curvature was increased. To avoid collisions between the bow and the soundbox, bridges rose, and their stronger push relies on thicker bass bars now. This raised some resonances.
Finally, many soloists prefer old Italian instruments because these are louder and more brilliant, a necessity to play in a big concert hall with an orchestra accompaniment, but these instruments don't sound nicer at all. Chamber musicians or Gypsies would comment "OK, loud and responds very easily, but sounds horribly, I prefer that other instrument" with a nice warm dark tone.