5:18 Scripture; This word will refer to the Torah, Prophets, and Writings.  For this is what was considered Scripture at the time.  The phrase about the ox is quoted from Deuteronomy 25:4. and the next phrase about the laborer, spoken by Yeshua is an explanation of it.   We must here distinguish between "Scripture" as the Word of the Almighty that is written, and Scripture as the word of the Almighty that is spoken.  Yeshua is responsible for the latter statement about the laborer, which was recorded in Luke 10:7.   Yeshua's words are certainly the Word of the Almighty.  The question is if Luke itself was regarded as "Scripture".  It is quite plain that anyone can quote the Word of the Almighty, but that does not automatically make the body of writing containing it Scripture.   This text neither asserts it nor denies it.  It is does not affirm Luke is Scripture, nor does it deny it.   However, it is clear that at a later time Luke came to be regarded as canonical Scripture, and that is how we treat it now since there is no reason to think that Luke was not inspired by the Holy Spirit in his collection and compilation of historical events.  Luke has proven reliable under examination and comparison with the Torah and Prophets.  

New additions to the Torah, Prophets, and Writings had to become accepted over time.  It was not automatically assumed by everyone that a writing was immediately viewed as canonical Scripture.