As it says, Garnet was reporting Stan's reaction to the song. Stan wrote it. Garnet merely sang harmony on the recording. Many songwriters have rued the day their lesser works became monsters (see Roy Williamson's "Flower Of Scotland")
But it is not Garnet's song, so noone should expect him to sing it. It seems to me that it took him a long time to get over Stan's death before he ever sang ANY of Stan's songs.
Garnet has written a lot of great songs of his own and is an excellent solo performer. If you compare vocals, Garnet has (now) a much smoother and fuller baritone than Stan (although, unfortunately, we will never know how much better Stan could have been) and is a better guitarist and multi-instrumentalist than Stan.
I think you do Garnet wrong to say he is less than his brother. He is a wonderful writer and performer who took a loss, grieved, redirected his life and moved on.
Why can't you?