Yep you're right, but at least that adds another layer of complexity to their attack. A lot of security controls are at least somewhat situational, and most non-draconian companies have a process to put further mitigations around those exceptions either from increased monitoring or adding additional supplemental controls.
There's no such thing at perfect security, just better risk mitigation. Slipping in a usb hub between the computer and keyboard while someone isn't looking is a bit trickier then just plugging in a usb stick. If you disable unused usbs in the bios, instead of trying to do silly stuff like glue them shut, then the attacker has at least been temporarily thwarted if they slot it into a dead port. Aside from the high traffic areas, disabling ALL usb ports in places like datacenters and especially colocated datacenters, can thwart the attack outright as well.
Really from looking through this thread a lot of people seem to be under the misconception that security that isn't perfect is pointless. It's like claiming that locking your doors is pointless because lockpicks exists. The point isn't to keep a sophisticated attack at bay, but rather to keep script kiddies and drive-by attacks from hitting your network. To defend against sophisticated attacks you really have to go a bit crazy, and even then very small slip ups can be disastrous. Ask Microsoft about their root cert getting leaked via a core dump!
I fully acknowledge that many people also work for places with dumbass security controls. Gluing usbs is WAYYYY up there on that list in my opinion. It also looks like a lot of people work at places that have really shitty security teams that haven't quite figured out that controls are situational and require more thought then, "see checkbox, execute checkbox."