No, it's easy to exploit.
USB host-to-host cables can be bought on Amazon for a few quid, or can be made using nothing more technical than two normal USB cables and a pair of scissors.
The software is easily acquired should one desire it.
Yes, you need physical access, but once you have that it's a privilege escalation right up to Ring Minus One - permanent, ongoing and irrevocable access to everything in the machine - simply by plugging your Raspberry Pi into an open USB port.
That's why it matters.
The reason it's no longer very serious is that a patch is available.