Growing a plant does not imply understanding soil minerals or photosynthesis. Training a dog does not imply understanding it's brain. Using LLMs to improve a codebase does not imply understanding C++. Improving a PC with new parts, does not imply understanding RAM.
You can understand abstractions about the system, without understanding the system. Without gradient descent or other forms of automatic optimisation, do you think humans could build something as capable as GPT-1?
Saying we understand training and deploying machine translation systems that have strong predictive (rather than descriptive) power, apparently has very little overlap with defining a formal, analyzable, mathematical model of language. Mathematician who refer to "understanding" generally mean the second thing, but the first thing is also a kind of understanding. Just messier.
Of course, ultimately that's because language itself doesn't follow simple rules like linguists would prefer. They're more like guidelines.