What do you mean “not completely true”? Do you mean it is harder, but only a little? Also, I pointed out that I don’t think it’s impossible or not worth doing, I just said that it takes more dedication and probably a decent reason (other than just “for fun”). You are giving just anecdotal evidence, too, and from a specific country (I’d venture that the need and/or benefit to learn multiple languages is more pressing in countries without English as an official language, too) [edit: It’s kind of unfortunate, too. People in non-English speaking countries are probably quite pressed to learn English and that might get in the way of someone who wants to learn something else, but needs or is pushed into the usefulness of English. Some people don’t have the time or willpower to learn a second language on top of that. So English speakers are pretty lucky in that regard. They don’t have to learn English as a foreign language, and the board is wide open]. Additionally, you have to consider whether those people started those two languages from scratch at the same time, or whether, for example, they took French 1, then started German 1 while taking French 2.
The case of OP is coming from English and starting two vastly different (from English) languages at once (I’m assuming OP level around N5 or lower).
Too lazy to look it up, but I wonder what the stats would be for time it takes to get fluent in two languages doing them one after another or together (so many variables though). For example, I wonder if you learn Japanese fluently in 3 years, then it only takes an additional year to learn Chinese fluently, whereas tackling both at once might take 5.