healthcare in italy is decentralized, to some extent. so while each of the 20 Regioni ("regions") must adhere to national rules, they have some room for action and can add specific restrictions.
Campania has consistently been one of the most severe regions when it comes to covid-19, with the president de luca publicly threatening to send soldiers to the home of people who don't respect the guidelines (which he did, at least on one occasion:
https://www.reuters.com/article/instant-article/idUSKBN23X1LQ ); he also quarantined a whole town, Saviano, after too many people attended a funeral.
you claim that de laurentiis used personal connections to get some doctors to send this letter which forces the players to quarantine: personally, i don't rule out anything, but for the reason explained above, the most logical explanation is that Campania simply keeps on being very strict, making no exceptions.
and in any case, as things stand, the players must remain in isolation.
juventus, on the other hand, are probably trying to force a win by default.