The way I see it is that since yields/spread went down, the price of the bonds went up, hence high returns for bond holders. However, for new bond investors, bonds are now pricey and have low yields so they are no longer an attractive investment (relative to higher return investment options)
"High return on corporate bond index last year implies yields and spreads come down, so bonds are less attractive now."
This statement is tricky to understand alone without further context (that probably the book provides).
Indeed, if a market bond portfolio outperformed in a previous period (due capital gains) because rates went down (so spreads), then it is less attractive now to buy bonds because they are more expensive.
For a bond index to appreciate to the level it is not much attractive to buy bonds now, it is probably because the volatility of real interest rate is higher. This volatility of REAL rates makes the bonds prices to change in a structural basis. Changes in the real rates generates changes in nominal rates.
If you hold a bond until maturity, then this effect does not affect you. However, the majority of investors does not hold bonds until maturity. In other words, they speculate over interest rates curves and the volatility of those rates.