What do do with German multi-word constructions
If I was learning German on Lingq, I would differentiate between words whose meaning can't be deduced from its parts and those that are simply a combination of two or more words just written together. So I would ignore your example Spielzeugauto because it would correspond to just "toy car" (two words) in other languages. It is only considered a single word because it is spelled without spaces. On the other hand, there are words whose meaning is different from the sum of its parts, as e.g. Bildschirm. Those I would lingq or consider known as individual words. Of course, the exact border between both is blurred but I would still ignore many compound words.
I routinely do the same thing in languages such as Russian. For example, I ignore compound words meaning things as "eighteen-year-old" or "twenty-five-storyed", and so on. The reason is that those words tend to inflate the known word count and it becomes difficult to estimate how many word families your lingq word count correspond to, which is something I like to know. The impact of such words seem to me to be particularly important in a language such as German.
If I know the meaning, I just mark them as known and move on. The composite word might actually a different meaning than just putting the two words together, as @ericb100 said, so it's better to check just to make sure.
I almost never ignore words, so, for example, I always mark names and the occasional English or French word as known as well. This inflates my known-word count a bit, but that's OK.
I think separable verbs are trickier to handle. There's a recent thread with good ideas on that here in the Forum.
I think compound words in German are individual words with individual meaning and I feel it's proper to mark them as known.
I just add these as known if I know them. English has many multi word constructions as well. They exist as separate words in the dictionary. (eg. playground) Why not count them? In the grand scheme of things "known" word count is just a metric to give *you* and idea where you are and how you're progressing. Although I can't think of any off the top of my head there are quite a number of German multi words that when the two or more words are put together the meaning is decidedly different that the two words put together...or at least in translation to english there may be a one word translation...eg. Kühlschrank...which is made of two words that translate to cool + cupboard/closet. The english translation is refrigerator. In English we would never call this the cool cabinet. Or we might say that it is a "cool cabinet" as in it's a an interesting cabinet.
IMO don't get too hung up on this. You can choose to ignore if you like. I ignore most city names and some countries. Others may count them. Not a big deal in the grandscheme of things. You might choose to ignore some multi word contstructs and others not.