Yes they are, any licenses are revokable. The GPL as well.
I could release software under GPL, not accept any patches, see the software grow and 2-3 years from now when a lot of people are using and depending on the software, bam I revoke the GPL from all previous versions and impose a special EULA in its stead. I can because I am copyright holder.
This is not true. None of the standard BSD licenses are revokable.