Ultrasound scans are able to tell if the fibroid is solid or fluid filled, so most of the time a biopsy is unnecessary, this is also the same for breast cysts or fibroids.
Yes, they use ultrasound for this but I am not convinced of what you can really tell with it. It is like watching a movie on a static filled screen. Making out images can be a bit of a problem.
The one thing that makes me wonder if they are right about it being a tumor is the size. That sounds as if it is awfully large for a cyst.