What the data on this site shows is that native people immersed in their L1, going to school etc still don't peak in their language abilities (at least vocabulary) until their mid-thirties
. So I always find it a bit bizarre when language learners think they speak like a well-educated native after 3-4 years work. If you can get to that level so quickly (with or without CI) then you should share the methods with the international school committees as apparently no school in the world is teaching their native language efficiently.
I think people are misreading what I said a bit. I am (admittedly self assessed) at C1 for reading and listening in German. So CI worked really well for me. The last time I looked a couple of years ago I had a knowledge of at least 10000 words in German, but that just doesn't equate to knowing all the words on a page. There are still lots of gaps as soon as you start reading serious newspapers or books, though I can still read and understand these things without a dictionary.
But it's still frustrating not being able to process German in the same way as English. I have been getting into audiobooks this year, and slowly cranking up the speed. I mostly listen to audiobooks at somewhere like 1.7x-2.2x normal speed, Youtube videos at 1.5x-2x (I can't listen to Youtube videos at normal speed anymore, people speak sooooo slowly). But if I try to do the same trick in German I quickly hit a wall. I want my German to be as good as my English, but that's going to take a long time.
And this year I have been doing a lot of English reading (after 5 years of only German - 103 books so far!) so I am probably getting slightly rusty with German. People over estimate the advantages living in their L2 community. I speak German every day (basically always when I leave the house), but I am learning little doing that as I have roughly the same conversations with the kindergarten teacher, baker, candlestick maker. I could presumably find a tandem partner I liked, but I am pretty certain one-hour tandem would give me less than 15 minutes reading a serious non-fiction book. The big advantage living here is that there is lots of culture at my fingertips (bookshops, films, theater, etc).