This part is actually FUD I think. This particular Dell private key does not chain up to a trusted root CA.
Also - Windows will only install drivers silently that are Microsoft WHQL signed - they are the only ones who sign these drivers, and this key does not chain up to that either.
At most you could sign a driver with this key, and install said driver onto a machine that had the public key already installed - assuming you had local admin as well - and for a user mode driver (like a printer) it will give you a soft warning "are you should you really want to do this", for a kernel mode driver it will give you a red "this will potentially harm/wreck this computer" warning.
Yes this is a terrible security problem, but the attack surface is relatively small (none of the Dell PC's I had - have this cert - I believe it only gets installed when using the support portal's check my serial/warranty feature).