I've never found this a convincing reason as to why a language is difficult to learn. Word order is learned once while vocabulary, speaking and listening need to be learned and refined consistently over many years. All of these become harder the further the target language is from the ones you already know. Would French really be significantly harder to learn if they said 'Je le ballon frappe'?.
I also think this applies to some other 'hard' qualities of languages, like pronouns and alphabets (i.e. not logograms). Conversely, freedom in how you approach grammar in a sentence makes it harder, not easier, because while you're able to structure a sentence how you like it makes it harder to know what sounds idiomatic and it increases the number of forms you need to listen and read for.
Tangential thought: one of the reasons that vocabulary is annoying can be presented in CS terms. Imagine reading a sentence like 'Do you want to foo the bar?'. From the context you can probably understand what kind of words foo and bar are but you don't know what they mean. Yet they're the most important words in the sentence. This isn't a coincidence: the least common words in a sentence tend to be those with the highest entropy. If there was another word that carried the same meaning but was more common, the author would be more likely to use it. So it's not enought to learn 90% of the words, you need the niche words too.