They advertise this as a network appliance, so it makes sense that they use Debian since being able to install packages via things like apt makes the whole experience much easier. A lot of career network engineers are not that savvy with sysadmin type roles, so something like buildroot would alienate their market a little
ARMv7, with 512M of RAM, isn't that embbeded and you win a full os whit ready to use tested packages, package managers, and boot system, it saves a lot of times and expertice, maybe they need to opt out of it if they want to have better termals, or more optimization.
I've had to work with embedded devices in the past (develop an alternative firmware for $THING), and irgh I hated buildroot which was the original firmware's base. Way too much effort to compile stuff yourself... a simple "debootstrap" and installation of your private packages is way easier to get stuff up and running. And when you run into some package you need, you directly run "apt install" on the device itself, add the install command to the script that creates your firmware image and that's it.
The best thing is, whatever issues you run into will already have been solved a million times before by the wide Debian community and whatever remains can be solved by a quick hop into #debian on whatever IRC network they migrated to.
