Rednote has been shown as the top free app (per Apple’s own App Store in my device at least) for going on a week, so the magnitude may be larger than you imply.

Also, having tried it myself, the algorithm works much like TikTok whereby it learns to show English speakers English content pretty quickly.

Also the general consensus among people who have used IG and TikTok (I personally don’t use IG) seems to be that the former does not at all substitute for the latter, particularly in terms of the subjective “authentic” feel of the content (IG often said to be lacking the community feel of TikTok).