If they use unmodified Linux, then they only have to provide (a link to) the source code to that kernel on request. No source code is required for proprietary add-ons, unless they are kernel modifications.

The GPL also does not state that the source code should be easy to find. In the early days, one had to write a letter, send it by mail, in hopes of getting a tape or CD-ROM with the source code. For which you then had to pay as well.