Is that true? This paper claims it is not.
Yes it's true, you can read my response to one of the authors @canjobear describing the problem with that paper in the comment linked below. But to summarize: in order to show what they want to show they have to take the simple, interesting languages based on linear order that Moro showed a human cannot learn and show that LLMs also can't learn them and they don't do that.
The reason the Moro languages are of interest are that they are computationally simple so it's a puzzle why humans can't learn them (and no surprise that LLMs can). The authors of the paper miss the point and show irrelevant things like there exist complicated languages that both humans and LLMs can't learn.