You didn't get it straight
I didn't say his opinion didn't mean anything but in general a main's opinion is going to hold more weight since they will have played the matchup more and know more about both their character and the matchup itself.
Player A is a main of character 1 and player B happens to be playing that character for this matchup too against player C who is playing character 2. If player A beats player C and player B beats player C then it
might mean character 1 beats character 2 but you also need to take into account how well player C knows the matchup. It may well be that both player A and B could be winning because player C doesn't know/play the matchup well enough. Sometimes it's a mix.