It tells us that Matthews is the better goal scorer. Period. Nothing elsei think a player being hands down better at every comparable year.... and then scoring 21 more goals this year. Tells us at least something?
Players develop at different paces. The NHL is full of examples like this. The Sedins, Draisaitl, Kucherov etc. Henrik Sedin, for example, had 42 points in his fourth year. Should we assume that another player with say 60 points in his fourth year will automatically go on to win the Art Ross and Hart trophy and have a better career?
It's flawed logic to assume linear development. Right now Mack is the better player. Could Matthews catch him? Possibly but he still needs to show that.