Comment by surajrmal
They didn't but keep in mind that the app is currently 170MiB. The standard library shouldn't have added more than a few hundred kilobytes. They already likely pay similar costs for c++, but it's more worthwhile as they have a lot more c++ code total.
Also note that if you statically link to the rust std library, lto will excise the majority of it anyways, no need to rebuild it.
The default hello world stripped with one codegen unit and panic=abort was 342kB both nightly and stable. Adding lto dropped it 42kB in stable and 40kB in nightly. Adding build-std and only building core did not reduce it any further in size.