Personally I agree that the numbers correlating to some kind of proficiency for inflected languages seem way too low for inflected languages, based on my own experience in Russian and in several non-inflected languages. However, I don’t think that the number of case endings/declensions in Russian and various imperfective and perfective verb conjugations in Russian (for example) are the main concern.
Once one has a basic knowledge of all the cases in Russian which should be somewhere at LingQ’s B2 level, then I would find it odd that intermediate students would make lingQs for regular nouns, adjectives and pronouns in each and every one of their case variations. Certainly I don’t and would only do so for an irregular form of a noun/adjective. I suppose I could, but if I understood the word in the given context (that is, in a specific case), what’s the point? To merely inflate the number of lingQs and then in turn “known” words? While some may do this, I personally care about what I understand (1) in a given context and (2) whether I know the word/phrase well enough to use it independently in conversation. The number of LingQs says nothing about the second which is my primary goal.
Similarly, for verbs, while in Russian there are imperfective and perfective forms for most verbs, in the Romance languages there are more verb tenses and six conjugations for each – that is, many more verb forms than exist in Russian. Again, I can’t imagine anyone making LingQs of regular verb forms in every conjugated form once one is at the intermediate level.
Yet Russian still has considerable challenges relating to grammar. It doesn’t merely have cases applied to nouns, but additionally to adjectives and pronouns which do not match in spelling the way adjectives agree with the nouns they modify in the Romance languages. Moreover, in Russian, different verbs and prepositions take different cases, with corresponding different meanings. Proper names and numbers are also declined. Which number is used affects the declension of the nouns that follow (e.g., 2,3,4 + noun is different than 5, 6,7, 8, 9 + noun). Thus, the difficulty is not merely that the ending of the noun, adjective and pronoun is spelled differently, but when that changes (some prepositions can be used with more than one case ending but this changes the meaning).
All this makes Russian grammar vastly more difficult for non-Slavic speakers. Learning the infinitive of a regular verb or the nominative case of a noun and adjective is easy. However, one cannot understand text or speech or say anything comprehensible without knowing which form of a noun or verb is appropriate in a given context.
Thus, for me personally, the “advanced” number of words I know on LingQ is not an accurate assessment of my ability to use the language. My skills in (1) reading, (2) writing, (3) listening, and (4) speaking are all different with my best ability being in reading and my worst, speaking. Right now, I am specifically working on improving my listening and speaking but that won’t be reflected in the number of words “known” on LingQ.
BTW, the US State Department’s assessment of words needed for proficiency in various languages counts “word families,” not individual words. Thus, all declensions of a single noun count as a single word as do all conjugations of a single verb. Verb forms of nouns are also a single word as are similar adjective and adverbs. How one counts this practically, I have no idea.
At best, the LingQ number probably correlates more to one’s reading comprehension and is motivational since one can see it increase. Yet to what extent one’s reading knowledge is applicable to other language skills remains dependent on the efforts of each learner.