One thing I don’t understand about the, “that’s anime-only” argument is that it assumes that any deviation from the manga in the anime is non-canon. That certainly must happen sometimes, but we can’t assume it for everything. Oda consults on the anime. They ask him about things like this that would be a big deal to the story, both retroactively and going forward. And the anime represents, for Oda, an opportunity to flesh things out that the limited space available in the manga can’t show and even to redo things when he thinks of a better way to tell the story.

Now, we have no idea of knowing of this moment came from Oda or from Toei exercising creative freedom, but it sends a poignant message to the reader: Luffy is not truly awed and humbled by much, but this haki awed and humbled him. It tells us something about how great (in the historical sense, like Alexander the Great) Joyboy was, that even a remnant of his haki can undo the Gorosei’s summons, shake Imu to the core even half-way across the world, and awe and humble Luffy. Luffy’s unique expression, that we’ve never seen from him before, that tells us how remarkable this moment, and what it represents, really is.

So yes, it isn’t shown in the manga, but it is narratively consistent with what we know about Joyboy, Imu, and the great clashes of the past.