Thanks for the answers. I was guessing it might have had something to do with the solo rules. It probably would have been better to mention it just in the solo or variant rules section and leave it out of the main rules. Or at least add a note about it not being relevant in multiplayer.
I think the generation marker is *very* relevant in multiplayer, as mentioned in the above threads. It's just technically not mechanically necessary.
There are two important roles it serves in multiplayer for me (even without draft):
1. A redundant tracking of who is supposed to be the first player. I can't tell how many times we forgot to pass the first player marker, and the generation marker saved us.
2. The value of most cards depends on how many turns are left in the game. The generation marker is a very handy reference of that that is much easier (though less precise) than analyzing the state of the board.