That would be an interesting argument. It is a bootloader (syslinux), the Linux kernel and an extensive userland, including dpkg and full set of root certificates. TBH, that is almost everything I need. I install a handful of additional programs, some static binaries compiled from personal source code archives, some programs from the Debian repositories, and I am good to go.
Perhaps it would come down to what exactly in the userland defines Debian as Debian. There are certainly differences in the rootfs as I go from one distribution to another.
It would be like arguing that NetBSD install media is not NetBSD. For me, it is absolutely NetBSD, just a smaller version.
You are describing the thing that is loaded and run to load the Debian installation data and install Debian.
One could argue the thing you are running is not "Debian", but Debian's installer tool.