Here is Mormon's conclusion.
What is most important is the end acknowledgement. Underneath the loaded words, the Lamanites are cultured and powerful. They have begun to "wax great." Of course they are still "cunning," and "delighting in all manner of wickedness" but of course that is because they are the enemy.
Begrudgingly, Mormon is telling us that they are to be reckoned with. Of course, all of the good they received from the traitorous Amulonites who gave them Nephite culture so they could begin to "wax great."
Mormon notes that they now "began to trade." Of course this must be incorrect. There is no way that they could have developed their civilization to the described extent without trade, if only for the necessities of warfare, such as obsidian. Trade is a part of virtually all societies. Donald E. Brown has examined multiple populations of the world to attempt to extract those things that we, as humans, hold in common. Among those things so prevalent among virtually all societies that they may be deemed a "human universal" is trade (Brown, Donald E. Human Universals. McGraw-Hill, 1991. P. 138).
That the Lamanites may have become even better at trade due to some of the knowledge from the Amulonites is quite possible. That they had never traded before this time is not at all possible.