has one USB port which will not connect to my Canon printer or my self-powered WD disk drive. Is this a fault in the Time Capsule, or not?
It is impossible to say without further checking, since it is possible and very likely that the USB port on the Time Capsule is performing perfectly.......and it is your printer which needs to be reset so that it will work with the USB port......and.....your hard drive may not be formatted to work correctly with the USB port.
So that is one issue......and I recommended that you take the Time Capsule to an Apple Store to let them check it out and test the USB port. If the port tests out OK....as I suspect it will.....then you know that you have a printer issue and a hard drive issue.....not a Time Capsule issue.
The seconds issue is, as I pointed out......that it appears as if an Apple "tech" might have misunderstood your question and given you an answer that did not apply to your situation.
or.....
If he really did tell you that the USB port on the Time Capsule was there only for diagnostic purposes, and it could not be used to allow a printer or hard drive to connect.......then he just flat out gave you the wrong information.....seriously wrong information....since he had the same Apple Support document on his computer that I provided to you. He was either too lazy to look for the correct answer, or he just made one up.
That is why I suggested that you talk to another and hopefully better "tech" when you visit again.
I will not comment further. Please let us know how your Time Capsule checks out when you can. Good luck.