When we're talking about potential alien lifeforms, assumptions like "fungus eat plants" need to be loosely held. This may not be fungi as we know it at all, but rather an alien analogue that fills a similar role despite having completely different origins. On earth, we know that evolutionary forces can "recreate" similar organisms independently. The same could be true between earth and alien worlds. Ichthyosaurs and Dolphins have a lot in common but aren't related [by earth standards]. Maybe there are comparable similarities between life on different worlds.