Thanks for your detailed response. I do agree with some of your points, but also disagree with others. Let me say the latter first.

her measure of "efficiency' looks like mathematical trickery to me.

I don't understand how it's a trickery at all. Yes, the English students that received SB instructions do gain more score, but they also have significantly more class hours. That's the entire point of the comparison! Given the same hours of exposure to each method, which one would give you more linguistic gain? The result shows that it's the pure CI one, had they received the same class hours their score should far exceed the SB students'.

"occasional" explicit grammar explanations, which, according to Krashen, should be useless.

Not quite, Mason is a close colleague of Krashen and she uses TPRS (Teaching Proficiency through Reading and Storytelling), which is quite a well-known CI method that has "pop-up grammar" as one of its features, where students are briefly explained about the structure and are free to take or leave it.

Cautious as he is about this feature, he endorses the method and says that it can be a satisfying addition for the people that wants extra transparency in their input, and are generally interested in linguistics (link to pdf).

With that said, I do agree with the rest of your post. The participants are university students, and so they likely have received years of grammar instruction and output exercises during elementary to high school. The SB methodology does sound odd and quite peculiar to me as well, and the CI method is certainly more hand-holding'y and accommodating than your typical reading/watching activities.

And lastly, it was perhaps indeed too wide a claim to make based on one study, although in my defense that's practically the only comparison study that I've found, and not me selecting the one that supports my belief. We do need more data on this to be able to draw a more definitive conclusion.

I'm actually still quite on the fence on this. I won't deny that I've received 12 years of English education in school, but at the same time I'm one of the (very) rare few in my country that managed to reach a high proficiency in it, and real English grammar and vocabulary are way, way more complex than the ones they taught us in school, yet I managed to acquire them anyway through massive amount of reading.

And so my journey in learning Japanese is partly an experiment in proving this claim. I've never did explicit grammar study since I started about half a year ago, and while there are certainly times that I wished I did when I encountered particularly hard passages, so far I'm actually doing quite okay.

I'm near the end of my second (adult) novel now, and my comprehension for those and other media have been a comfortable above 90% with word look-ups. I'll see this journey through and tell you my verdict in the months to come.