Exactly right. One way to know the Sabbath is moral rather than ceremonial is the Sabbath was established in Genesis 2:1-3. That means the Sabbath pre-dates the Law, and even pre-dates sin. So Adam and Eve would have kept the Sabbath before the Fall, and so would have all their posterity if they had never fallen.
As confirmation of that idea, Exodus 20:11 states that the reason God gives the fourth commandment is because the Sabbath is a creation ordinance, and by implication is therefore moral.
Adam and Eve had no conception of the Sabbath - they never worked or labored for anything prior to leaving the garden. God walked and talked with them daily, whenever apparently - what need for their to be to recognize that which is accessible and available all the days.
What purpose would a God have at all for the life inside his creations to set aside 1/7 of their time "for him" anyways?
As confirmation of that idea, Exodus 20:11 states that the reason God gives the fourth commandment is because the Sabbath is a creation ordinance, and by implication is therefore moral.
I hope that helps! God bless.