My guess is that it means some parts of the library are under GPL, some
under LGPL, and some in the public domain. If that's the case, the
library as a whole must be considered to be under the GPL, correct?
Not necessarily. If work is done on the Public Domain portion of code,
and the author wants to continue releasing changes to that portion under
the same license, he or she may do so without "poisoning" it with GPL.
The same is true for the LGPL portions of the library. GPL doesn't
conflict with LGPL or Public Domain in any way.