That's not entirely true. For the most part, say 99% are, but there are also a few graphically independent alphabets that aren't traced back to Egyptian hieroglyphics, but they were all invented after the idea of an alphabet and for the most part are relatively modern (Zhuyin, Ol Chiki). So it's really hard to call them unique inventions. In addition, there are few alphabets that people constantly debate over (Ogham, Hangul, Tāna) in terms of if they are actually derived from the Proto-Sinaitic alphabet or not. However, in many of these cases there are conceptual (not graphical) links to letters.
Also, it should be noted that there are separate lines of ancestry descending from Egyptian hieroglyphics. So that Meroitic and the Proto-Canaanite, although both descended from hieroglyphics are two graphically independent inventions of the alphabet. (Since hieroglyphics themselves were not an alphabet).
Finally, there are instances where several ancient cuneiforms actually used phonetic representations (particularly in Elam and Akkad).
So you have to make the distinction between the graphical alphabet and the concept of the alphabet when thinking about it as a unique invention.
(Braile is a good modern example of this. It is not graphically descended from the Proto-Sinaitic group but clearly conceptually linked).
Conceptually there is a good chance everything can be traced back to Egyptian Hieroglyphics, only because the few graphicaly unique alphabets were often invented in direct response to a Proto-Sinaitic derived alphabet.