I read this as AVH getting pissed off about people maybe questioning why if Ed wrote all the music did Alex get writing royalties and Michael Anthony get his taken away.
Dave by all accounts wasn't there at the point that the music was created, Alex is implying that when he jammed with Ed he was making significant contributions to the song writing. I think it's more likely he was contributing arrangements which aren't part of music royalties but as he says, we weren't there.
I still think it could be argued that since rock songs are made up of 3 parts - music, lyrics and vocal melody Roth should have got half and Ed half. If he wanted to share his 50% with his brother then Ed could.
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