I don't buy the "checking system was lax" excuse for one moment. It's just that, an excuse, and a pretty lame one.
It was always obvious that someone else had decided which historical figures were going to be talked about. How much is scripted we'll maybe never know. I'd even go as far as saying that Nakky might not have done some (or any?) of the drawings herself.
Then rewatching the episode I noticed that in some parts you can hear the crew laughing at some bits. So we have a scriptwriter (or if not full scripts, ideas on who to talk about) person, camera crew, sound crew, editors, etc. And not one person realised that it might not be a good idea to broadcast it?
Well here's what I think happened. But first to back up for a moment... Here in the UK we've always joked about Hitler. Even during the war itself there were radio shows that joked about Hitler, and various comic songs such as "Hitler has only got one ball, Goering has two but very small, Himmler has something sim'lar, but poor old Goebbels had no balls at all" and "Whistle while you work, Hitler is a twerp, wiped his bum with chewing gum and shoved it up his shirt." When you turn someone into a figure of fun it helps to make them less powerful.
However in Germany, talking about Hitler is obviously a bit of a taboo. Some people, particularly the older generation are ashamed of their countries involvement in the war and don't like to talk about it. That gives rise to the Fawlty Towers episode where some German guests were staying at the hotel and the phrase "Don't mention the war" is used. And of course Basil Fawlty did mention the war, with hilarious consequences, and as a Brit I can laugh at Hitler because we've always laughed at him. I saw the Fawlty Towers episode "The Germans" when it was broadcast for the first time on German TV in 1993 and somehow it made the episode even more funny. I'd be genuinely interested to know from any German members of the board if I've got that right, is Hitler really a taboo subject in Germany or are you able to joke about him? Maybe a generational thing?
Now onto Japan. I'd also heard that Japanese people don't like to talk about Hitler because of Japan's incvolvement in wwii. Now seeing that Yorosen episode that clearly isn't the case. You've got the kids firstly, who probably haven't been taught enough wwii history (I had heard that Japanese schools teach a very biased/edited version of world history, please correct me if I'm wrong) so they can plead ignorance. Then you have the camera/technical crew. Obviously some Japanese people can and do laugh about Hitler as we heard them laughing during the Hitler bit. I'm guessing it's mainly the older generations of people in Japan who don't like to talk about the war.
So... I'm assuming that what happened was TV Tokyo thought that attitudes had changed enough that they were able to have a laugh about Hitler without anyone complaining. Sadly they were wrong.