the word συριακη is Greek for "Aramaic" and the LXX translator's note is apparently referring to an Aramaic copy of Job that was used instead of or along with the Hebrew text.
The use of Aramaic is understandable because the Hebrew text of Job has many unique phrases and is a strange sort of Hebrew. An Aramaic copy of Job is noted in four other places in antiquity. There are two copies of an Aramaic Job found at Qumran, one little snippet, and then a long text with characteristics that show that the translation was imported from the East. There are also two rabbinic stories concerning the first century Gamiliel family that revolve around a tranalstion of Job and most would assume that that was an Aramaic Job.
Job is the first and only whole book of the Hebrew Bible that is attested in Aramaic in the pre-Christian era. The little piece of Lev 16 at Qumran may have been either from a pilgrimage reading for Yom ha-kippurim or from a translation of Leviticus.
the Aramaic translation called Syriac Peshitto is a Christian era translation and not the background for the LXX Job.