Are Italians genetically separated from other Europeans?
I'm afraid that the "genetic island" idea is almost certainly wrong because


The wikipedia page offers no evidence for this claim:


  Following scientific research carried out by Dutch geneticists, Italy has proven to be one of the last two remaining genetic islands across Europe (along with Finland), this due to the presence of the Alpine mountain chain that, over the centuries, has prevented large migration flows aimed at colonizing the Italian lands.[citation needed]


Remember that any random idiot person can edit a wikipedia page. If there is no supporting reference for a claim you find in wikipedia, you should treat it as suspect.
As you point out, the Alps only block entry on the north of Italy. Italy, however, is a pretty large peninsula, completely open to the sea along its entire length. For example, here is a map of Greek and Phoenician colonies circa 530bc (source):



As you can see, significant parts of the coast of what today is Italy were colonized by Greeks and Phoenicians. There were many subsequent waves of migration and/or invasion by various tribes such as


The Lombards (image source):


The Visigoths who sacked Rome in AD410
The Ostrogoths whose kingdom in its prime contained most of modern Italy





The many Jewish communities in Italy that go as far back as the Roman era.

An article by Brisighelli et al. (which is actually quoted at the very top of the Wikipedia page you linked to) mentions that (emphasis mine):


  A total of 583 individuals were sampled from across the Italian Peninsula, from ten distant (if homogeneous by language) ethnic communities — and from two linguistic isolates (Ladins, Grecani Salentini). All samples were 
  
  [...]
  
  Italy shows patterns of molecular variation mirroring other European countries, although some heterogeneity exists based on different analysis and molecular markers. From North to South, Italy shows clinal patterns that were most likely modulated during Neolithic times.
  
  [...]
  
  Before the Roman conquest, ancient Italy was characterized only by the presence of Indo-European populations living in the Italian Peninsula since the second millennium BC, corresponding to the period between the Iron Age and Romanization. During all this period there were also increasing contacts with the Phoenician and Greek colonists: the former being largely present on the coasts of Sardinia and western Sicily and the latter in Southern Italy.
  
  [...]
  
  At the beginning of the first millennium BC the following native tribes could be distinguished on the Italian territory: the Ligures, on the coast that bears their name, in the northern Apennine valleys, part of the pre-alpine valleys and the western Po Valley; the Sicani, in the interior of Sicily; and the Itali, in present-day Calabria (from whom comes the name ‘Italy’, which was to be extended to all the territory of the peninsula). Besides the already mentioned Terramare tribe, on the southern edge of the Po Valley, and the Villanovans, probably from Eastern Europe who settled throughout Central Italy, there were also the Umbrians to the east of the upper basin of the Tiber. The Veneti, who occupied the territory that still bears their name, originally came from Illyria as did the Messapii (now modern Salento or South Apulia) and Iapyges, who settled in present-day Puglia (Apulia) 5. Many other populations of Central-Southern Italy were created by the mixing of local and foreign elements dating back to the previous millennium; it is the case of the Sabines and Latini who settled in Lazio together with Falisci, Aequi, Volsci, Hernici and Ausones. The interior of Abruzzo was dominated by the Vestini, Paeligni and Marsi, while the central Adriatic coast was populated by Picentes, Marrucini and Frentani. The Apennine area of Molise and Basilicata was peopled by the Samnites and Lucanians. In Calabria and Sicily there were also the Bruttii and Siculi.
  
  [...]
  
  Molecular indices indicated that most of the Italian samples show diversity values that are comparable to other European populations. However, some differences were shown to exist, especially in isolated Ladin populations. 



References


Brisighelli F, Álvarez-Iglesias V, Fondevila M, Blanco-Verea A, Carracedo Á, et al. (2012) Uniparental Markers of Contemporary Italian Population Reveals Details on Its Pre-Roman Heritage. PLoS ONE 7(12): e50794. doi:10.1371/journal.pone.0050794