Could a small breast lipoma have come back as benign breast tissue on core biopsy pathology?

Yes. Lipomas are fat. Fat is part of normal breast tissue.
Yes. When they take the core biopsy, the sampling device passes through normal tissue on the way to the lipoma. But the lipoma itself should not contain breast tissue, it is a fatty tumor. So the biopsy results should have some of both tissue types. It's also possible that the imaging test was wrong, and it wasn't really a lipoma.