Whatever you linked to on imgur doesnlook very reputable. Eusebius simply relates in P.E. that the Egyptians believe Thoth gave them the alphabet. There is no attempt to link Thoth to Adam as in the text you linked, who, again, seems like some pretty disreputable nonsense from reading just that small sample. Eusebius later in P.E. says that the Phoenicians or perhaps Syrians, who he seems to wrongly equate with the Hebrews, likely invented it. He also seems to take the Cadmus myth as fact.

The origin of the Phoenician/proto-canaanite script and its ancestor, the proto-sinaitic script, is almost certainly a set of Egyptian hieroglyphs that represented individual phonemes, or else hieroglyphs whose first Semitic a translated phonemes were used. Wikipedia has a convenient table that shows one scholar's hypothesized equivalences here:

https://en.m.wikipedia.org/wiki/Proto-Sinaitic_script

These particulars of the history and development are still under investigation, but it has long been widely-believed with great confidence that the Phoenicians adapted hieroglyphs somehow or other, and that the Greeks, through the early Euboean trade activities in the northern Syrian coasts, brought it westward, mistakenly adding vowels and giving rise to the modern alphabets (Encyclopedia Britannica, Fox Travelling Heroes, and Grant The Ancient Mediterranean and The Rise of the Greeks)