Look, I will give you the REAL reason why your parent's visa was rejected. It was NOT because of "reasons" etc. It was.... to put it mildly due to "wisdom" of IRCC encoded in a software called Chinook. Read more about it here : (
https://vancouverimmigrationblog.com/tag/chinook/). Chinook is used in a number of counsaltes including Canadian counslate in China and in India.
It among the other things, auto-generates the message of rejection. This is the reason why lately rejection reasons make so less sense. They are auto-generated garbage.
Chinook allows a Canadian embassy staff to "bulk-reject" applications that fit a certain criteria and generate rejection message which is based on that group and may not be true to the actual case.