We are not told exactly what the 70 elders saw other than the fact that they saw God and what He stood on. This is one of numerous instances of epiphanies in the OT where people saw "God" in various forms and circumstances. Here is a sample:
- Gen 18:1, 10 - Then the LORD appeared to Abraham by the Oaks of Mamre in the heat of the day, while he was sitting at the entrance of his tent. ... Then the LORD said, “I will surely return to you at this time next year, and your wife Sarah will have a son!”
- Gen 32:30 - So Jacob called the place Peniel, saying, "It is because I saw God face to face, and yet my life was spared."
- Ex 3:5, 6 - “Do not come any closer,” God said. “Take off your sandals, for the place where you are standing is holy ground.”d Then He said, “I am the God of your father, the God of Abraham, the God of Isaac, and the God of Jacob.”
- Josh 5:13 - 6:2 - And the LORD said to Joshua, “Behold, I have delivered Jericho into your hand, along with its king and its mighty men of valor. (V2)
- Judges 6:14 - The LORD turned to him and said, “Go in the strength you have and save Israel from the hand of Midian. Am I not sending you?” [See also V16]
- Eze 1 - the prophet's vision of God; many elements of which are repeated in Rev 4 & 5.
... and so forth. Note the large diversity of "forms" in which the visions of God appear.
Now, we are also told that:
- 1 John 4:12 - No one has ever seen God [the Father]
- John 1:18 - No one has ever seen God, but the one and only Son, who is Himself God and is at the Father’s side, has made Him known.
- Isa 64:4 - From ancient times no one has heard, no ear has perceived, no eye has seen any God besides You, who acts on behalf of those who wait for Him.
Thus, it appears that these epiphanies in the OT were likely the pre-incarnate Jesus.