That is not true at all, they were advancing.
One thing that should be remembered, is that today most anthropologists believe that North American Indians were the first to use copper. Actually predating the use in Eurasia by at least 2,000 years. So quite literally, while those in Europe, Asia, and Africa were still using at most stone tools and weapons, a lot of the Americas were already using copper. Primarily because North America has the largest deposit of native copper on the planet. So literally all they had to do was pull it out of the ground and beat it into shape.
However, that same easy to use copper was also long term the problem, as they never had to develop the refining or smelting techniques that were used in the rest of the world to actually convert copper ore into a useful end product. And without that, they never looked into alloys, like bronze. There were a few cultures that actually did develop bronze by 1500, but it had barely made the leap from jewelry to tools, and not really to weapons yet.
They were advancing, but much more slowly as they had a much lower population density. And they had other barriers that slowed their progress. Like the aforementioned large amounts of native copper, so they never had to learn about smelting and refining raw ore. Also, the very noticeable lack of any kind of animals that could be domesticated into beasts of burden. The rest of the world had camels, horses, cattle, donkeys, and more that could be domesticated into beasts of burden. The Americas had none of that, other than in South America where they had the llama.
Lack of any animals that could be readily domesticated other than chickens and ducks also did a lot to hold back their agricultural and technological development. That is why I often laugh when people wonder that they did not have the wheel. Oh, they had it, but only for toys. Because quite literally, without an animal to pull a cart like oxen or horses, what use is a cart in the first place? Why would anybody invent something they could not use?