The link that you posted is a bit out of date, but the information in the article is correct.
You can connect an external USB drive to a Time Capsule (or AirPort Extreme) if the following conditions are met:
The drive is formatted in Mac OS Extended (Journaled). Most store bought drives are pre-formatted for Windows in a format called NTFS which will not work the Time Capsule. You have to use Disk Utility on your Mac to re-format the drive to work correctly with the Time Capsule.
Use a powered USB hub as the USB port on the Time Capsule is a bit under powered and it will not supply enough current for some drives. As a general rule....use a powered USB hub....even if the hard drive has its own power supply.
At times, I have had 3 drives from differerent manufactures all hooked up simultaneously into a powered USB hub which in turns connects to the USB port on the Time Capsule. No problems at all.
There are never any guarantees, but if you follow the guidelines, chances are good that your drive will work just fine.