MacOS Mojave installer out of date W.T.F.?

No - its for "Can the certificate be updated" only.

and YES for "I don't have to download the whole .... installer again?"

He can use it, by changing system date.

https://www.macworld.com/article/3097533/installing-os-x-what-to-do-when-this-copy-of-the-install-os-x-application-cannot-be-verified.html

Invoice for my wasted time incoming!

You are not addressing Apple here. We are end users like you.

if you don't want to download new installer, then change the system date before Oct 22, 2019. Then you can run installation. Its better do it in Terminal by typing this:

sudo date 1022100019

or

date 1022100019

(it means 10/22 10:00 am 2019)

Then you can install with old certificate. This work for all outdated installers with old certificates.

No.

Did you make a bootable USB installer ?

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12