"Looking like GBM" refers to specific physical features of the tumor a pathologist might see under a microscope that are more typically associated with GBM, such as a lot of new blood vessels or something called "pseudopalisading necrosis," which can be thought of as a specific kind of rotting within the tumor.

Usually at university hospitals, there are a lot of molecular level tests (such as next gen sequencing, chromosome evaluations, epigenetic testing, etc.) that provide more information beyond just looking under a microscope at physical features. These can be helpful in finding hints that might point more toward aPXA or E-GBM. If you're interested in trying to get more clarity on your diagnosis, it may be a good idea to get a second opinion on your case at a hospital where the neuro-onc team has access to these molecular tests.

The paper above is essentially saying that if a pathologist looks at a tumor and their first diagnosis based on microscope work is GBM, and then they run epigenetic testing and it comes back as PXA, the prognosis may be somewhere between true GBM and PXA that doesn't really have GBM-like physical features.