Don't think that matters. I've written (from scratch and inherited) and deployed many ClojureScript frontends, from one page ones with lots of interactivity to 30+ pages/sections, none of them reaching the size of 2.5MB minified (when using the production settings for Closure Compiler). Add in SSR and/or code splitting and the weight should be nowhere near there, leading to the guess that something is wrong in their config or they are embedding binary files into the JS asset.
Okay, so how about not inventing outrageous ideas on the spot and just consider what is written?
It's equally as hard to unload what's been boiling in your brain for last half a year. I'm reading comments and trying to decide where to expand my post, of course, but this "binary" thing just got me laughing.
EDIT: I mentioned webpack since the thread is about react, but you can do the same thing manually by putting the base64-encoded data in a data URL on a CSS file.