Comment by adrian_b
This is a modern distinction.
Until recently the term "alphabet" was used for any writing system where the symbols correspond approximately with phonemes, regardless whether both consonants and vowels are written as in all alphabets derived from or inspired by the Greek alphabet, or only the consonants are written, like in other writing systems derived from the old Semitic alphabet, without passing through the Greek alphabet.
Then the term "abjad" has been created, and also the term "abugida" (for alphabets where the base symbols are for consonants and the vowels are added as diacritic marks around the consonants), and the sense of "alphabet" has been restricted, in order to distinguish these 3 kinds of alphabets, but "alphabet" in the older wider sense can still be encountered frequently, either in the older literature or in informal speech, so one should be able to recognize both the stricter and the wider meanings.
In TFA, "alphabet" is used in the old wider sense. Moreover, it is not even used correctly in that sense, because they did not find a written "alphabet" like those used in teaching, but they have found a few written texts that are believed to have been written using an alphabetic script.
The oldest actual alphabets that have been found (which show the alphabetic order of the letters) are for the Ugaritic alphabet, which is older than the Phoenician alphabet, but much more recent than the oldest inscriptions that are believed to have been written with an alphabetic script.
And we live in the modern world, so we should use the modern definitions?
For reference, "abjad" was introduced in 1990, so it's 35 years old. I don't think it's unreasonable to expect people to start using 35 year old terms to accurately describe academic subjects.