"And the LORD said unto Moses, How long refuse ye to keep my commandments and my laws? " (Ex. 16:28)
This verse suggests they did know about it. But because of slavery in Egypt and being surrounded by idolatry they had neglected or been forced to stop observing the Sabbath.
On Mount Sinai a written code was given, but before that the Patriarchs knew the principles of God's law. Abraham kept God's commandments Gen. 26:5. Joseph refused to commit adultery, Cain murdered Abel etc.
Exodus 16:28 does indeed sound like they might have known about it before this incident, if stripped of context. We read earlier in chapter 16:
22 On the sixth day, they gathered twice as much—two omers for each person—and the leaders of the community came and reported this to Moses.
23 He said to them, "This is what the LORD commanded: 'Tomorrow
is to be a day of rest, a holy Sabbath to the LORD. So bake what you want to bake and boil what you want to boil. Save whatever is left and keep it until morning.' "
Saying they knew about the sabbath before Exodus 16 is not supported by these verses.
The sabbath was given as a test to see if the COI would follow commands given by God.
4 Then the LORD said to Moses, "I will rain down bread from heaven for you. The people are to go out each day and gather enough for that day.
In this way I will test them and see whether they will follow my instructions.
5 On the sixth day they are to prepare what they bring in, and that is to be twice as much as they gather on the other days."
Apparently this resting was a
new test, from
new instructions. They had no clue about resting on a sabbath. This contradicts the assertion that the COI were going to observe the sabbath in Exodus 5.
People knew about morality before Sinai, but the ceremonial sign of the weekly sabbath rest, given to the nation of Israel as a covenant, was never mentioned before this.
Edit to add: isn't it stretching the imagination a liitle to think that trying to get the COI a one time, three days vacation to worship is related to a weekly sabbath observance?