Jesus referred to what Moses said as "the word of God " because God spake by Moses even as he did by the former prophets, and likewise through Moses he also said
Duet 18:18 I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him.
Even as Jesus said,
John 5:46 For had ye believed Moses (which were also Gods words written) ye would have believed me: for he (( wrote of )) me.
But God says by Moses... and will put my words in his mouth; and he shall speak unto them all that I shall command him.
Jesus, confirming the same words God spake by Moses (which were written) and saying in accord
John 12:49 For I have not spoken of myself; but the Father which sent me, he gave me a commandment, what I should say, and what I should speak.
Jesus confirms what God has spoken even as is also written by Moses ( as Jesus apostles also confirm the same is Jesus in Acts 3:22 & Acts 7:37)
God (speaking by Moses) as is written isn't finished, God continues, saying
Duet 18:18 And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
Its twofold in the rejection but that which is spoken (which is also written) is the one judging him
John 12:48 He that rejecteth **me**, and receiveth not **my words**, (( hath one )) that judgeth him: the word that **I have spoken**, (( the same )) shall judge him in the last day.
Or "the words of" the Word made flesh (whose words are not His but He that sent him) thus the words/ word of God (as spoken) even written
Same with building upon a rock (which is Christ, or the sure foundation) consists of both hearing His words (which are not His but He that sent Him) and doing them (which were the words spoken by Christ and also written)
We have God speaking by Moses bearing record (or witness of) another whom God would raise up (like unto him) who would speak the words of God, even as this speaks of Jesus Christ speaking the words which were given him of God, even the Father (even by commandment). Jesus referring to the words by Moses as the word of God, given God spake by Moses and Moses writes of Jesus who would be given the same (the words of God) both agree in the same.
I don't see how "being written" somehow diminishes the words spoken, or that somehow it had more weight in His mouth (back then) then it would now (because its written on paper).
The words to be given (even as recorded in scripture) by Moses (as Moses wrote) find their fulfillment in God putting His words in Jesus mouth (which He is commanded to speak). Jesus says, this is one that will judge him even the very word (that the Word made flesh) has spoken. Even as these are written.
Which (as I regard them) as being scripture being God breathed, even the word/ words of God (as spoken by Jesus Christ, the Word made flesh) are written for us .
Me and my words. Who is the Word (made flesh) speaking the word/words of He that sent him (as it is also written as prophesied beforehand). How do you even separate them? Since the Word (made flesh, Jesus Christ) speaks the words of God (whose words/ doctrine was not His).
And likewise Jesus said,
John 17:18 For I have given unto them the words which thou gavest me; and they have received them, and have known surely that I came out from thee, and they have believed that thou didst send me.
Same with them as it was with him
John 12:44 Jesus cried and said, He that believeth on me, believeth not on me, but on him that sent me.