That's misleading even if you're not a follower of Stallman. Lots of operating systems use Grub that aren't Linux-based.
There's a substantial difference between grub password protection, a feature which I've never seen anyone use in practice and the ubiquitous login prompt.
The OP specifically said the login prompt was defeated by backspacing alone.
Reminds me of the bug in Solaris where you could get root by essentially pressing entering a username and mashing the keyboard a bit:
Not really. You can use Syslinux in place of GRUB (in fact, I was when this bug was first appearing in the news). I also think it's bad practice to generalize things to that point. There is of course the obvious issue of Linux just referring to the kernel, making it hard to talk about just the kernel if you get into the bad habit of referring to many things by that same name. More importantly, though, if there's a bug in GNOME or your file manager, it feels very wrong for someone to say it's a problem with "Linux". I think this distinction matters more due to the modularity of GNU/Linux and all the choices of programs for basic tasks.
Tell that to Richard Stallman, who will go ape if you call GNU tools “Linux”