The earlier, unknown, abjad, must have been used to write the same 27 consonants as in the Ugaritic alphabet, but using graphic signs similar to those used by the 22-sign Phoenician alphabet, which resemble in form the Egyptian signs.
There is no doubt that the Egyptian writing system was the inspiration for the first abjad, because both the method of writing only the consonants and the direction of writing were inherited from the Egyptians.
In Ugarit, in order to write on earth tablets, like in Mesopotamia, the original graphic signs used for the 27 consonants were replaced with cuneiform signs.
On the other hand, the Phoenicians deleted 5 letters, because their language was simplified and the 5 deleted consonants were eventually pronounced identically with other 5 consonants. It is supposed that the Phoenician pronunciation was simplified so much because it was used as a lingua franca for commerce, by many people.
This reduction in the number of consonants created later problems for other Semitic people, e.g. Hebrews and Arabs, who still pronounced distinctly some of the consonants that were deleted from the Phoenician alphabet, so they had to invent diacritics to mark the missing consonants (e.g. shin and sin in Hebrew).
Although the Ugaritic and Phoenician alphabets come from the same area, they are discontinuous. However, the ordering being roughly the same, and following an earlier Egyptian ordering, is the big hint that they share at least inspirational descent from the same source.
Why that area of Lebanon? Byblos has been an Egyptian colony in that region for a very long time (~4600 years ago), ensuring continuous scribal presence. Scribes of that region are noted for intense multi-lingualism (as perusal of Ugaritic tablets is evidence of), so cross-pollination of scripts makes a lot of sense.
I haven't read through Goldwassers papers, but a lot of what they quote in the article seems to be unnecessary to explain the transmission of the abjad.
The difference is just that 5 consonants are missing from the Phoenician alphabet in random places: between gamma and delta, between kappa and lambda, between mu and nu, between nu and xi, and between sigma and tau.
It is known with which other consonants the deleted consonants were merged. For example Ugaritic had "s" and unvoiced "th" like in "thin", while Phoenician pronounced both as "s" and kept only the sigma letter.
(I have used the names of the letters as used by the Greeks, as those are more familiar for most people)
The Ugaritic alphabet also used for certain purposes 3 supplementary signs, invented later, and which were added after the original 27 consonants.
I've no idea how well that story is supported, but abusing new technology for purposes it was never intended to fill does seem like an eternal human fact.
[0] https://en.wikipedia.org/wiki/Mater_lectionis [1] https://en.wikipedia.org/wiki/Phoenician_alphabet#Greek-deri...
My understanding is that it was an abjad, not a syllabary. An abjad has only consonants, not vowels. When a word is written, only its consonants get written, and the vowels are only implied. Hebrew and Arabic are abjads.
The Phoenician alphabet was preceded by the Ugaritic "alphabet", which was also an abjad.
A syllabary usually has many more symbols than an abjad or alphabet. If n denotes the number of phonemes in a language, then a syllabary has O(n^2) or O(n^3) symbols. In contrast, the Phoenician abjad has fewer symbols than even the English alphabet.