IMHO, Visitor Visa validity (which could be a maximum of 10 years or up to the validity of passport) and Visitor stay extension/visitor record (which could be a maximum of 6 months) are two separate things.Why would you need a visa extension if your visa is still valid? Do you mean for example, they have to exit every 6 months but the validly goes beyond the 6 month mark?
I'm not sure about that to be honest. My understanding is that a visitor record is to be applied for as an extension after visa expiry. Check the link:
https://www.canada.ca/en/immigration-refugees-citizenship/services/visit-canada/extend-stay.html
The link you mentioned is for Visitor stay extension, but I didn't find anything for the Visitor Visa extension. I am still wondering if I have to apply for a new visitor visa application.
Hopefully one of our experts may chime in on this one