There was a long discussion recently about the portfolio's of so called TBM funds. viewtopic.php?f=10&t=108922
The portfolios are quite dissimlar. Moreover the Bar Cap Agg Index itself has changed markedly since 2008 making historical comparisons less useful.
BTW Fidelity Total Bond FTBFX is not a TBM Index
fund. The Fidelity index fund is Spartan U.S. Bond Index Investor FBIDX.
A scientist looks for THE answer to a problem, an engineer looks for AN answer. Investing is not a science.