Their C++ style has a heavy emphasis on pointers. In addition, their banning of exceptions warps their code in other ways (for example a heavy use of out parameters).
Also, they have a tendency to use their own alternatives to stuff in the standard library.
If you already know modern C++ you can discern the good advice from the bad/archaic advice, but if you are learning Modern C++, you would be best served by avoiding it.
In any case, the simple fact that the reasons are unknown speaks pretty heavily for the case that the reasons aren't universally valid, so you would be learning a dialect of C++.
Google documented pros/cons and reasons for their decisions in their style guide, which is public [1]. Whether or not Google's final decision is applicable to you, reading the pros and cons is informative. For example, their reason for not using exceptions is specific to their existing code base [2], but the discussion is informative.
[1] https://google.github.io/styleguide/cppguide.html
[2] https://google.github.io/styleguide/cppguide.html#Exceptions