If it's not headcanon, then it's merely heavy speculation, which isn't much better. A character having information on the VS Battles Wiki mentioned and regarded as a possibility means that there is at least some sort of implication to it that could be given the benefit of the doubt. Evil Leafy only has that for the creation of her internal world. There isn't information to give her the benefit of the doubt for regarding the multi-solar system level power being combat applicable. Your reasoning allows a character to get a possible attribute they don't have any evidence of having. I currently have her ranked a top tier along with Black Hole and Four, (which I hope doesn't get changed,) so Evil Leafy is still acknowledged to be superior to most contestants.
I am best at explaining with examples. Evil Leafy doesn't have evidence of her multi-solar system level power being combat applicable, just like she doesn't have evidence of her internal world being universe sized. There being a lack of evidence doesn't mean we can overstate the capabilities of the character. Evil Leafy doesn't have evidence of her multi-solar system level power being combat applicable, just like Firey doesn't have evidence of being able to shoot fire out of his fire body, and we should not assume the possibility of either character having either abilities. Something not being deconfirmed still doesn't mean we can regard it as possible if it has no evidence.