In Chapter 30 he talks about his strategy for taking money out of his accounts. His goal is to maintain a 75% stock : 25% bond ratio across his entire portfolio. He reminds us that bond index funds belong in the tax advantaged bucket, and in particular IRAs, not Roth IRAs. He then shares that as he approaches the time when he will need to start taking RMDs, he is selling off his assets in after tax accounts (stocks), not assets within his tax advantaged accounts because he wants the latter to continue growing in the tax advantaged environment so long as possible.
So, if I am interpreting him correctly, he is selling off assets in the after tax accounts (stocks) and as such, the rebalancing will involve _selling_ not buying bonds as he gets older.
I realize within his tax advantaged accounts, his stock index funds are probably outperforming the bond index funds, but it is nevertheless a curious result that contrasts markedly with conventional wisdom, i.e. that one's allocation in bonds should increase with age.