Why would anyone assume Exodus 20:11 about Sabbath observance would require YOM in Genesis 1 to be 24 hour day?

God established time for man, not for Himself. Sometimes a week is referred to as a year, but never as a time period. A day is as a thousand years and a thousand years as a day in God's time, means there is no time in God's calendar. If God just haphazardly gave no meaning to time, we would be very messed up.