They are only using material for which they have a license (at least debatably). Open source software licenses usually require attribution if you reproduce the source code or use the source code in a program.

Some other uses are allowed without attribution. Someone can read and learn from open source software without needing to put an attribution anywhere. You could run an analysis of the code on GitHub to find out what percent of code is written in C++. You wouldn't need to attribute every project on GitHub.

Now the debate is whether this applies to training ML models.