Greenwald alleges NSA tampers with routers to plant backdoors

Glenn Greenwald, the journalist responsible for funnelling many of Edward Snowden's revelations to the world, has penned a book in which he alleges the NSA intercepts routers before US manufacturers can export them, in order to implant backdoors.
Excerpted by The Guardian, Greenwald's tome No Place to Hide alleges the following:

"The NSA routinely receives – or intercepts – routers, servers and other computer network devices being exported from the US before they are delivered to the international customers."
The agency then implants backdoor surveillance tools, repackages the devices with a factory seal and sends them on. The NSA thus gains access to entire networks and all their users."

Greenwald's source is a 2010 NSA document.
If true, Greenwald's allegations mean the USA is perpetrating just what it accuses China of doing. Which isn't very good or nice.
An argument could be made that the USA alleges Huawei's ties to China's military and government mean its back doors are installed at the factory, a level of complicity said to make Huawei unworthy of trust.
Yet if these allegations are correct, all manner of US companies must also be complicit. How else could kit shipped through normal channels be diverted into the NSA's hands long enough for it to be tampered with? With transport and logistics companies swearing by their delivery times and freight aircraft hardly likely to wait for one missing parcel, surely someone, somewhere, is helping to make this happen?