Sardinia was never, ever considered Italian in the first place, even by Dante who’s widely regarded to be the father of the Italian nation (whatever that means).
Historically, culturally and linguistically speaking, Sardinia was already a “nation” on its own, even if split into four independent kingdoms, with its own culture and language (later languages), long before the concept of a united Italy had to be theorized.
Alas, it didn’t take much time that the indigenous states gradually lost their independence, and Sardinia was forced to endure a long, very long period of foreign occupation, first by the Catalan-Aragonese Crown, followed by the Piedmontese-Italian House of Savoy on 1720, after acquiring the island at the end of the War of Spanish succession. So, no will by the population to join neither Italy, nor the Italian process of unification, rather some sort of a resignation to the following events.
The Savoyards themselves were not pleased with the acquisition of Sardinia, since they exchanged Sicily for it and, as far as this may sound ridiculous to you, they even tried to sell it, and failing at it, on 1860 (see Giovanni Murgia’s historical papers and research).
However, as confusing as this may seem to you, the Kingdom of Sardinia was a Kingdom seeing Sardinia as basically a colony, again, where to send down bad people (proof of that is the common Italian saying, later popularized, of “ti sbatto in Sardegna”), and to steal some valuable raw materials from, like wood for instance. The real center of the Kingdom was only Piedmont, located in Turin.
Every Sardinian attempt to gain independence failed miserably, so there was little to do about it but to lower one’s head, living under the last (in the West) European feudal regime. In the meanwhile, under Piedmontese rule, every indigenous sign of the cultural heritage, even the language, was literally wiped ouf of existance and, nowadays, very little of it remains (but still stands).
To put it simply, Sardinia was radically italianized, very much like Ireland was anglicized. The rest is known, since we Sardinians had to and keep to share our history with the recently founded Italian state. Apart from a couple of things, among other stuff: Sardinia still has the highest number of jailhouses in Italy, serving to keep most of the mafiosi coming from all of the Italian mainland, and last but not least, over 60% of the Italian and US military installations are located here, just like a European Okinawa (American readers will probably know what group of islands I am referring to, by saying that).
So, it is very nice of your Italian acquaintances to label Sardinia as “one of the most Italian of all the regions in Italy”, especially when you have a fairly reachable place of summer vacation that is now able to speak your language very well.