Totally possible that I read it wrong...it's under the Applying to Change Conditions or Extend Your Stay in Canada guide:
It reads:
"Starting on April 1, 2011, you will be able to work in Canada for a maximum period of four years. Therefore, you will need to start counting the time you work in Canada as of April 1, 2011...If you do not work during the entire specified period on your work permit (for example you have a work permit valid for four years and you are sick or you leave temporarily Canada), you may need to submit proof of time not worked when you apply for another work permit later on and you are close to the four-year maximum...
Note: It is your responsibility to keep track of any breaks in work while in Canada on a work permit. You will be required to provide documentary evidence to prove periods where you have not worked in order for that time not to count towards the four-year limit"
I guess I'm just uncertain as to whether this clause applies to a PGWP that was receieved prior to April 2011
No, this wouldn't apply to a PGWP. This only applies to those who are trying to show that they did not already work the maximum time allowed under the new TFW rules.