Apple recommends 400ms for startup time because that's how long the app open animation takes. It's also hard to tell from just one flamegraph how long it will really take in prod, that's why the percentages are nice to use. I've mostly used percentiles like p95 to track and prioritize mobile perf work. 600ms on one phone could be 2s on older hardware with other factors slowing it down, but it depends on the distribution of doordash users.
I actually came across the 400ms number in a 2019 WWDC talk while researching the topic! It said 100ms of that time was delegated to the iOS system for setup, leaving 300ms for whatever your app does.
Granted the 100ms section included System Interface - DYLD3 (dynamic linking) as part of it, which includes the 200ms portion that was optimized in tfa so I don't know how accurate that 100-300 breakup is.
The talk also says to avoid dynamic library loading during launch to improve time which is interesting, as it suggests there's an alternative, which may or may not be what Doordash did here.
