Earlier quoted context omitted.
You cannot submit you application except using the toolchain that was released today, which is occasionally different enough from the beta tools that your app will no longer work. So now you have a broken app and just hours to fix it, and then somehow App Review is going to get through it while a thousand other people are submitting at the same time you are? No way.
Well you can always ship updates with the old iOS 13 SDK toolchain (Xcode 11) the entire time. You could have tested on the iOS 14 betas, and if you found a crash, you could try to implement a workaround and resubmit in the weeks and months leading up to today. Sometimes your crash is because of bad assumptions in your code, or sometimes your crash is due to real Apple bugs creeping into the iOS APIs. Not saying it's…
When I was an iOS developer, this is exactly what I did. Install iOS beta, find any show stopper bugs, fix them in the current SDK and get the bug fix out. Most of the time it was just the beta OS highlighting an existing bug that the current OS ignored.
Once the bugs are fixed, then start adding beta OS features. These features often have to be gated anyway since it's normal to want to support current version -1 or -2.
So yeah, Apple should have given more than a day, but it's not nearly as serious as the hot takes want to make it out to be.