This might be another seed planted to connect dots - subs and underwater caverns/cave entrances.
The Chinese issue is hotter right now than Eggstein's island. This would be a good way to plant the possibility of such a thing happening in normies heads, but will then re-inforced once more on Maxwell's shenanigans come up. Maybe, could be.
https://www.dailymail.co.uk/news/article-8645191/Satellite-images-capture-single-Chinese-nuclear-submarine-entering-Bond-lair-cave-complex.html