The actual load per page was around 1-5mb, so even when doing remote mobile testing in areas with low speed internet it wasn't an issue (although its definitely one of the main reasons to switch to a real react site when scaling the product)
I think someone else said there are other optimizations that happen when you shop production code.
Aside from that, they use the app literally their entire workday, so spending 5 seconds loading the thing isn’t going to hurt anyone.
It can be slightly hacked around to add user authentication & analytics, so if you are handling a small amount of users its a really efficient way to iterate fast.
Of course the initial users will experience a slow product, but the idea is to discover faster what a useful product looks like without investing development resources.
But let us be realistic. I just loaded www.amazon.com - 17.8 MB transferred when it finally stopped. Arguably one of the most successful websites in the world.
A 5MB asset size for most sites is perfectly acceptable.