Wrong. You can print from Mac, but not from iPhone.
The thing is, AirPort Express/Extreme only wirelessly extends the USB cable. You need a matching printer driver on your Mac to make it work.
iPhone has no printer driver(s). And AirPrint is a driverless printing protocol.
There are AirPrint activators available, however. You need to keep it running on a computer for as long as you need your AirPrint printer visible on iOS devices. That way, your computer pretends to be an AirPrint printer and will send it's output to a printer that's been configured to print on that computer.