> won't achieve any sort of fluency in your target language from taking a course in it

For a randomly picked person, yes, but it doesn't matter to apps that specifically cater to motivated language learners. It's like saying "new training shoes will not make a random person run better or become healthier".

Obviously talent and motivation are mandatory.

However, any stock-market valuation right now is divorced from the success potential of the business. That's a separate issue.