Open source often uses the analogy of a river/stream - in that there are upstream projects, which can flow down to those downstream.
Linux (kernel) is upstream of Debian, as it is of GNOME project, KDE project etc.
Debian is upstream of Ubuntu. Ubuntu is BASED on debian-testing, though it'll likely include parts of debian-unstable as well.
Linux Mint is further downstream that Ubuntu, as it uses Ubuntu as its base.
Yes, future versions of Ubuntu will reflect changes being made currently in debian-testing or debian-unstable, but a Debian release has NO impact on Ubuntu really, mainly because the relevant changes have already made it into Ubuntu.
Example: MATE desktop maintainer for debian is Martin Wimpress. He's also the (head-)maintainer for Ubuntu MATE. Martin makes changes for Ubuntu, I've heard him say, by making them in Debian, and they make their way into Ubuntu anyway. It saves him heaps of work with almost no duplication. (Ubuntu-MATE & Debian with MATE desktop are not identical; Ubuntu MATE contains a few extra things only in Ubuntu MATE).