They are hyperbolizing :-).
They are saying that the new iPhone is great (which they will of course say) and that the new one is the best thing that has ever happened in "the iPhone world" since the first version was released. That is, while many people may have produced many great apps, and while iPhone 1, 2, 3 & 4 were no doubt great, the new iPhone 5 is far better than any of the rest of them.
It does NOT mean "that the first occurrence of iPhone happened to itself" or that "the first iPhone happened to iPhone."
The phrase "the greatest thing to happen [to] since ..." is a standard phrase that means that something is "extremely marvellous" or modern or innovative. The literal meaning is related to this meaning but not what the users ears/brain are intended to hear. ie the expression is not analysed or parsed - it just conveys the same standardised meaning any time that it is heard.
A once common variant was "The greatest thing to happen since sliced bread".
Up until about the 1950's bread was almost invariably sold as an unsliced loaf. Pre-sliced represented a radical change in usability and standardisation of slice size and was used as a means of conveying new / marvellous / innovative / fantastic ...modern.
Using Google's Ngram Viewer I compared the frequency in Google Books of the segments -
The result was "interesting".
It may be that the renewed and ongoing increase in "sliced bread" references may be due to some late 1950's wit having coined the GTSSB phrase and as it became the default expression it carried "sliced bread" with it.
However, if you want the real winner of the "since xxx" stakes but NOT the "greatest thing since" stakes, you are offered for your valued consideration "since Adam". Note that "greatest thing since Adam" hardly registers.
It appears that "sliced bread" and "since Adam" may approximate a zero sum game, but it seems more likely that this is a good demonstration of the old saw "correlation does not prove causality".
But to put things in perspective.