There are limits based on the size of the binary that apple will allow on the app store. In addition, here's a really interesting story about how uber had to fight to downsize the app cause they realized that staying under the cellular data limit made people able to download the app when they were on the go: https://twitter.com/StanTwinB/status/1336890442768547845
I used to run an iOS testing-as-a-service thing. Few of our customers came close to the App Store limits. The ones that did were games and they are definitely not looking to optimize apps for the long term. We did have some unicorn customers but they weren't doing 2GB downloads, for the reasons you mention.
So how many customers could this startup conceivably get? Unless you're a game with lots and lots of graphical assets, you have to be the kind of company with more ability to employ developers than to coordinate them (i.e. a unicorn).
It's also a moving target as limits are continually revised upwards. I assume they have done some diligence about their TAM, but I don't get understand the plan just yet.
