I think others pointed out in this thread as well, that bunpro seems to use a hybrid approach, context plus strict grammar concepts, I think this latter what you understand under teaching grammar from first principles. Turning off hints in this approach seems counterproductive, because without context, you remain with the primary principles, or not even that.
From what you wrote, chunking seems to be somewhere in the immersion field, however, the approach does not seem to be perfect for the following reason: you learn specific grammar concepts in a specific context, while in real life communication you face an unlimited number of possible contexts. This is why context and immersion in all areas are paramount to me. Without context and added immersion, the result would be stunted.
Comparing early childhood language acquisition with adult learning is misleading in my opinion, because the two work in very different ways. I learned my mother tongue very differently than I learn languages now as an adult and I would likely die of frustration if I had to learn the same way as a baby/ small child.
As adults, we have a framework to build on. We have understanding how certain things work, we can sit down and learn 1000 words… have you ever seen a 4 year old sitting down learning 1000 words in their own mother tongue?
I mentioned in an other thread discussing books that children speak their mother tongue(s) proficiently at their own stage. A 4 year old child can speak their mother tongue perfectly as appropriate for their age, even if they do not speak academically or do not know the usual average number of words in their own language.
While to me chunking and learning elements of speaking/ language usage seems to fit into immersion, I do not agree that it is the most effective learning method. At least not for me, as chunks do not help me when I want to communicate what I want in real life, if the situation does not fit the bracket of what I learned. There is a finite number of chunks facing a possibly unfinite number of real life situations.
To extrapolate from chunks designated for specific situations to cross-use them in a different context, presupposes a high level of language usage/consciousness, which starts after intermediate level, at least for me. I sincerely hope everyone else has all these ready puzzles in their heads waiting to be used and they can pull the right one the right time from the very beginning. From some examples in this forum, apparently it is not just me who usually rather pulls a blank.
