I don't think this is entirely true. It feels almost like a no true Scotsman: if even parts of a task can be related to anything in the training data, then clearly it's not novel enough for the sake of this argument.
It feels like language models are becoming more capable of picking novel problems apart, identifying strategies for the pieces, and then executing the pieces to come up with a complete solution. This is part of what makes CoT reasoning pay off so well.
I am interested to see what comes after CoT reasoning (perhaps the latent-space reasoning [0] that recently hit the front page?), and how it'll affect which problems can be effectively solved with a model. But I feel like it's not very useful to say models completely fall apart on novel tasks.
Maybe they fall apart when the solution includes completely novel strategies, but sometimes models engineer new strategies too, so even that's not completely off the table.
But then again, you could still say that if it's possible to just engineer a solution out of the blue by recycling patterns from the training data, was the problem ever even novel in the first place?
Also consider that models don't only directly learn patterns from the training data. There's a limited degree of generalization that enables the model to reproduce the patterns of the training data, but not necessarily only the patterns of the training data. The way loss is modeled in training is designed to allow the model to plausibly output the training data, not to ensure the model outputs nothing else.
[0]: https://news.ycombinator.com/item?id=50145337