I am convinced that this is true, but I don't know why. Why?
I would say memory is a very important factor here. People may have better or worse memory, but in general it's very easy to forget what you learnt a week ago, if you didn't repeat it in the meantime.
It may and probably does apply to other fields of study than language learning, but to language learning, it seems, for some reason, particularly relevant. Why?
In most subjects you can relate things you learn to other things you already know, and that makes them easier to remember. In the case of language learning at the beginning you need learn completely arbitrary words and constructions. Of course, it gets easier after a while, because you begin to have an intuition how the new language works. For the same reason, learning is easier if you already have foreign language learning experience, or if the new language is related to some of the languages you already speak. But in general, when you learn a language, there many things you need to remember that are completely arbitrary.
I think this notion encodes why beginners often give up languages, and is closely related to the concept of frustration. Enabling "momentum" at the beginning is therefore important, but axiomatically almost impossible. How can it be done?
Something that works for me and for many people, is spaced repetition. In my experience, it's important to get at least passive understanding of at least 1000 most frequent words in a language in the shortest possible time. And since there is no one-to-one equivalence between words between different languages, it's best to learn them in the context of sentences. Of course, that's not the only thing you need to learn as a beginner (depending on the language, some knowledge of the phonology, morphology and syntax of the language may be crucial), but it's very important. When you know the core vocabulary, the language stops being completely arbitrary, and you can relate things you learn to the ones you already know. And it makes the new words and constructions easier to remember. At this stage the intensity becomes less important, and even with 2 hours per week some progress is possible.
The advanage of using spaced-repetition systems (SRS) is that it can be very motivating: you know that in a short time you can get to the level where you at least have a general idea about a lot of what is written in the language. It's also resistant to breaks. It's most effective to study every day, but what do you do when you had a few day break? Normally it may be hard to decide, if you should just go on with new material, or go back and repeat what you've learnt. SRS make that decision for you, and it's usually an optimal one.
But SRS is not for everyone. For some people, SRS (and flashcards in general) are simply too boring, and even if they know it's effective, they can't be bothered to spend their time on it. They probably need to use other methods, but no matter how it's done, getting familiar with the core vocabulary remains very important.