Keeping Jehovah’s commandments and statutes meant keeping the law of Moses. The statement that “they should keep the law of Moses as yet, for it was not all fulfilled,” is almost certainly Mormon’s text. Since Mormon lived after that fulfillment, he was more aware of the differences than Alma the Younger would have been. Alma the Younger would have kept the law of Moses and looked forward to the Messiah, but would not have understood the ways in which the gospel of Christ superseded the law of Moses.
There is no chapter break at this point in the 1830 edition of the Book of Mormon. Therefore, there was no division between verse 17 and the next verse, Alma 26:1. Verse 17 is the introduction to Ammon’s discourse, and sets the stage for it. It would really be best to read this verse as being part of the next chapter rather than the end of this one.