Nothing in the game says they aren't, either.
That's the point of all this; logically, it doesn't make sense that Cloud could fit into a six year old's panties. But as a game that already had its fair share of logic holes, a concept that was abandoned halfway doesn't fall into the category of logic.
It is unlikely that there was any point in this idea that they would straight up tell you whose panties the mystery panties belonged to. The joke was most likely going to begin and end with that one scene. Any extra information would be gleamed from each player's own interpretation.
And since it's a fact that Tifa lives with a child and likely handles her laundry, then there is still that small window that suggests that the stolen panties do belong to Marlene. Just the mere idea is enough to make the whole thing incredibly uncomfortable, and that is most likely the reason Square removed this scene in the first place.
And again, even if it was a hundred percent confirmed that they belonged to Tifa, the image of Cloud wearing the stolen panties of his childhood friend/love interest is only marginally less squick than the above concept. Either way, it skirts the line (ha!) more than anything else that was left in the game.