> Urheberrecht explicitly states, that downloading something is legal

Technically right with your phrasing, but wrong considering the context.

That exception is explicitly about the fact that eg pictures served on a website will be copied to the local cache and can still be shown without violating it. But besides that, nobody argued that the download itself was the license violation.

If Roblox only downloaded and then deleted it without using it in any way, they'd not have violated the license. Was that the point you were making? If so okay, that's true but from my perspective a pretty stupid view. Surely you're aware that they didn't?