Idiotismo (no connection to idioto) is Esperanto for "idiom" (a false friend with Portuguese "idioma"). An idiotismo is an expression that you can't understand from it's component parts. OldBen44 is saying that if you're calling someone "freshly baked" that nobody would be able to understand that it means that they're new at something.
Lol, I was a bit confused by his comment because idiotismo is a Portuguese word which, among other meanings, in fact has to do with idiots. I've never been a big fan of idioms, so that might be a good way to remember idiotismo hahaha.
Well, the meaning of "freshly baked" seems to be more easier to figure out than something like "it's raining cats and dogs".
I saw a great comment in the German forum about how questions like "Why does German do X?" will slow down learning. I wish I could find it. It was a good answer - avoids the appearance of defensiveness (often attributed to similar answers given for Esperanto) and shows that Esperanto isn't the only victim of this thinking.
That said, I'm sure it's orders of magnitude more common for Esperanto. My favorite is when a new learner (and I've been there myself) carries on about a "foolish choice by Zamenhof" only to slowly catch on over months or years that most European languages do it that way and English is the odd man out.
I have some concerns about your comment. First of all freŝbakita in this sense dates to well before Duolingo and even before WWII. Duolingo has no power to change this expression. Freŝbakitaj esperantistoj is part of Esperanto. (It's a common expression, actually.) If you have chosen to learn Esperanto, it will be necessary to learn this expression.
Secondly, I think your joke about gas chambers is in bad taste. At least, I hope it was just a tasteless joke. If not, then your comment is even worse. It refers to newness - newly created. It's a positive expression.
In sweden the expression is "nybakad" and has been in use since at least 1715, that's looooong before both ww2 and duolingo. And we still use it... (If you can read swedish...) https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=2ahUKEwjrptr-pr7iAhVOlIsKHRmoBeYQFjAAegQIBhAB&url=https%3A%2F%2Fwww.pressreader.com%2Fsweden%2Fspraktidningen%2F20181218%2F282196537042701&usg=AOvVaw1BSQR_6W1rIectVhGLNRyF