It depends on the heat and material and time. A brief touch on a metal object is lightly to cause a superficial burn. Hold you hand in boiling water for 4 or 5 seconds and see if you can shake that off in a week.
But more honestly, McDonalds admitted that they new many customers drank their coffee immediately after purchasing it, and that it was unsafe to drink as served, and unsafe to transport in a moving vehicle, and that their competitors accounted for this and acted responsibly.
Because you know what? When I get a drink, to drink, I expect it to be drinkable. I don't expect it to be in a condition that it could conceivably send me to the hospital for a month involving surgery. Especially if it was actually significantly hotter than drinks I was used to purchasing.
So, combine that: McDonalds knowingly took actions they knew would result in serious but preventable injury to at least several hundred people a year, accepted those injuries as the cost of doing business, and then refused to settle and pay that cost. In addition, many of their competitors DID act responsibly, reducing the likely number of injuries and dealing a pretty big blow to any claims they had justifying their actions. I'm honestly not surprised they lost.
Personally, I never buy hot drinks at McDonalds at all, in large part because I don't want to wait a frickin' half hour, with the cover off, running the risk of a major spill thanks to traffic turbulence, before I can drink it. (Well that and the fact that their stuff is super watered down, way to big, and tastes terrible. But meh.)