Not really true.
Does it seem odd to anyone else that playing just your level 50 earns you more Legacy xp than playing your alts?
It is true that you can gain more Legacy XP for doing content with your level 50 character than with a level 1-15 character (if that wasn't true, people would create and destroy throw away characters to optimize Legacy XP, which we really don't want).
We want Legacy to be a viable feature for both players that enjoy in alts and players that do not. So the features and system are balanced to give benefits to both types of players.