Yep, and there’s no API for Vanguard. You could in theory automate the re-balancing otherwise, and do it more frequently as well.
Edit: I’ve found an old interview from John Ameriks (the head of Vanguard’s Investment Counseling and Research Group), who says that the funds rebalance daily!: Interview with Vanguard’s John Ameriks about Target Retirement Funds — Oblivious Investor