Can the Arduino Eclipse plugin v2.2 be used with Arduino 1.5.8?
V2.2 is pretty old by now.
As far as I recall it should work fine.

V2.2 of the Arduino eclipse plugin should work fine with the Arduino IDE version 1.5.8. 
What that phrase wants to say is: the plugin V2.2 was released before Arduino IDE 1.5.8 was released; as such the code can not make a statement on the compatibility. 

But again V2.2 is old. We hope to release V3.0 end of February 2016