If, as the parent comment claims, the library has been relicensed more permissively then it is irrelevant that it was previously only licensed under the GPL. Though it is certainly bad form to continue distributing it with that notice.
There's a bit of a myth that if you release a program containing GPLed code then you irrevocably get the obligation to release your source code whether you want to or not. Certainly you're in violation of the terms if you don't, but releasing your source code is not the only way to correct the violation. Even if you were successfully taken to court over the matter (which can only be done by the copyright owner of the GPLed software, since it's a contract with them that you're breaching), the court might require another resolution e.g. that you remove the GPLed code from your software or pay the copyright owner a fee/fine.
There's a bit of a myth that if you release a program containing GPLed code then you irrevocably get the obligation to release your source code whether you want to or not. Certainly you're in violation of the terms if you don't, but releasing your source code is not the only way to correct the violation. Even if you were successfully taken to court over the matter (which can only be done by the copyright owner of the GPLed software, since it's a contract with them that you're breaching), the court might require another resolution e.g. that you remove the GPLed code from your software or pay the copyright owner a fee/fine.