The Pharisees had many oral traditions for how to keep the law that they traced the command for back to Moses. Many thought that it was impossible to keep the written law of Moses without knowing their oral traditions for how to keep it. For instance, they reason it was impossible to keep the command not to work on the Sabbath without knowing what counted as work, they gave a higher priority to their traditions, which Jesus criticized them for (Mark 7:6-9). In any case, they would never have considered teaching someone to keep the Sabbath without also teaching them their traditions, so that was all wrapped up in their concept of what it meant to keep the law of Moses.
So what was rejected was that Gentiles had to keep their many oral traditions and that they needed to keep the law of Moses in order to be saved, but they did not reject God's holy, righteous, and good law.
Acts 5:29 But Peter and the apostles answered, “We must obey God rather than men.
This theme is consistent throughout the Bible where man's laws are overthrown while God's laws are upheld. It's almost bizarre to consider teaching people to obey God's commands to be going against His will.