To me those seem like to sides of the same coin. LLMs are fundamentally trained to complete text. The training just tries to find the most effective way to do that within the given model architecture and parameter count.

Now if we start by "LLMs ingest huge amounts of text", then a simple model would complete text by simple memorization. But correctly completing "234 * 452 =" is a lot simpler to do by doing math than by having memorized all possible multiplications. Similarly, understanding the world and being able to reason about it helps you correctly completing human-written sentences. Thus a sufficiently well-trained model that has enough parameters to do this but not so many that it simply overfits should be expected to develop some reasoning ability.

If you start with "the training set contains a lot of reasoning" you can get something that looks like reasoning in the memorization stage. But the same argument why the model would develop actual reasoning still works and is even stronger: if you have to complete someone's argument that's a lot easier if you can follow their train of thought.