I'm a working man, and I don't always have time to respond to a post. Be patient.
The phrase does not exist in scripture, no. But just because a phrase is not in scripture does not mean that the concept or idea which the phrase describes is not in scripture.
Did Jesus exist before He was born?
Have you ever considered that Jesus is God the Son? Meaning there is one Person who is the Son, not two.
The author of that page must not know anything about Hebrew grammar.
The suffix "-im" indicates a plural noun. If it did not, then "cherubim," "seraphim," and "nephilim," would not be plural.
The author also apparently doesn't know any Greek either, or he would know that there is no Greek word for "Elohim," so the closest word was used, "theos," which without the "ho," just means "a god," or "gods" (lowercase 'g') (depending on the form of the word).
Yes, singular verbs are always used when Elohim, a plural masculine noun, is used. However, why would Moses, in the first book of the Bible, in the first chapter, the first verse, use a plural masculine noun to describe God if He is not a plurality? Why not use "El" or "Eloah"? Did Moses make a grammatical mistake? I doubt that.
As for Deuteronomy 6:4, the word "ehad" is used instead of "yahad" or "bad." "Ehad" means "one of plurality," whereas "yahad" and "bad" mean "one of singularity." The same word is used in Deuteronomy 6:4 as in Genesis 1:9, 2:24, 11:1, 11:6, 22:2, 32:8, 34:16, etc. When talking about God, neither "yahad" or "bad" is used, ever (as far as I'm aware). It's always "ehad."
"sema yisrael YHWH elohenu YHWH ehad"