Earlier quoted context omitted.
I still think it's interesting that, even though the violinist knew which violin he was playing, the audience liked him playing the Opus 58 violin better than playing his own Stradivarius, which he could have been subconsciously influenced to play better because of its familiarity.
He could have been subconsciously influenced to play the Opus because of its novelty.
(I come from a family of professional violinists, and very nearly became one myself, so I am at least tangentially qualified to comment on this.)
The difference between one violin and another is very noticeable. The shape of the bridge influences the distance between the strings and the height of the strings over the fingerboard. Violins have idiosyncratic resonances, a very loose sizing standard (my Vuillaume feels much larger than my sister's Mermillot), differing shapes (both subtle and obvious---see "guitar-shaped" Strads, for example), and varied neck angles and thicknesses, to name just a few off the top of my head.
Beyond that, professional violinists practice and play a lot---hours each day. To say that he would be more familiar with his own instrument is a drastic understatement.
On the other hand, none of this is proof that somehow the Opus 58 is better or worse. There are many superb-sounding instruments that are nevertheless not as celebrated or as highly regarded as the Strads and their ilk, and the reasons have little to do with their tonal quality. For example, who wouldn't want to play on the same instrument once owned and played by Paganini?
For that matter, there are many old makers whose violins are considered at or near par with the Strad, e.g., Andrea Amati and Guarneri del Gesu. These instruments share with the Strad a rich back story that no modern instrument, no matter how superb a specimen, can have. In other words, they're not just great instruments, they're literally some of the centerpieces of musical history.