Does it make sense to only mention amp requirements and not voltage requirement, or vice versa?
You are right in that both voltage and current must be specified for a DC power supply.  However, sometimes one or the other is omitted because it is implied.

In the case you quote, note that it mentions "The Raspberry Pi is powered by a micro USB".  That implies 5 V.  The only remaining question is now the current.

As another example, consider a device that has a power cord attached ending in a wall plug compatible with outlets in your locality.  Brief documentation for that device might only say that it draws 1.5 A.  Since it obviously runs from line power, it might not explicitly say "120 VAC, 60 Hz" (or whatever the line power is in your locality).

More thorough documentation someplace should specify the voltage, current, and any other relavant parameters.