Here's what I think happened -and that's why I asked if the application was submitted in April (I may of course be wrong): IRCC can only extend visitor visa/visitor record applications for 6 months at any one time. So, regardless of the duration of your parents' initial visitor visas, they are only allowed to stay in Canada for 6 months at any one time (you most likely already know that). Since you applied in April, the extension was granted for another 6 months ending in October. IRCC counted the number of months for the extension beginning from the time (April 2018) the application was submitted, and not from the time (August 2018) the application was approved. Since your parents have now stayed in Canada for more than 6 months, they are required to undergo medical tests. They were not granted the extension to enable them complete the medical exam. Instead, they were instructed to go for the tests as that's required by (immigration) law.
I would advise you to take them to a panel physician for the medical exam, book their flight tickets to return to your home country where they are/should be ordinarily resident and have them come back to Canada after living there for an extended period of time (at least 6 months as earlier suggested). I am only advising you to do that NOT because I fear that they would be refused another extension if you apply again say in October 2018, but only so that whoever will consider the next visitor visa/super visa application would clearly see that they still meet the definition of visitor - you can't have them live in Canada for a whole year or two and still convincingly submit an application claiming they're just visiting.