So, the flashcard group was just using flashcards as a supplement to extensive reading.
Well, at the end of the treatment period, both groups had the same vocabulary retention. But a couple months later they tested again, and the "no flashcards" group had better retention of the target vocabulary. And not by a small amount, either.
It's not terribly hard to find examples of second language acquisition researchers expressing skepticism about flashcarding, though. I don't necessarily find all of it universally convincing (if I did, I wouldn't have 99 reviews left to do today in my δΈζ deck), but I do have to admit that a lot of the evidence people claim in support of using spaced repetition systems for language learning has the look of pseudoscience. It largely consists of taking some very limited, tightly focused lab studies, and massively over-extrapolating from them.
But I wouldn't want to over-extrapolate from the studies that claim flashcarding doesn't help, either. The protocols often focus on a very minimalist implementation of flashcarding that's easy to study. Or the protocol is really designed to test some theory about how memory works and not some theory about study strategy. (This second example describes basically all of Ebbinghaus's work, by the way.) One could easily argue, for example, that using a flashcarding application to rote memorize a pre-made deck of words and phrases that somebody else chose is a fundamentally different practice from sentence mining your input materials.
While learning Thai -- which is a difficult language already -- I continually struggled with word definitions that should have been easy. How hard is it to remember the words for the various parts of your head and body? With just word definitions, apparently, it's really, really hard.
But then I supplemented this with other tools. One teaches a few words, then throw those words into full phrases in little quizlets. You pick out the written phrase which you think matches the spoken phrase. It always includes one or some of the words you've learned. It puts the words in context, cementing the definition, teaching you grammar, and "foreshadowing" new words. It's skyrocketed my retention and comprehension.
Pictures also help. Children's books with audio narrating are huge.
I guess my point is, Anki with word definitions alone is definitely garbage, or less than ideal at best. Which is why most language learning resources stress that it alone isn't enough - and most learning resources who push Anki (or SRS in general) don't say "do only this." It's part of a balanced breakfast, so to speak. Incorporate it into your workflow, but don't make it the workflow.