Could anyone please explain :“a 30-year, option-free, corporate bond and an MBS with a 30-year legal maturity with the same coupon rate are not equivalent in terms of interest rate risk”
I understand that MBS with a 30-year legal maturity is the same as 30-year CALLABLE BOND

Not exactly the same, as the MBS has a prepayment option, not a call option; the prepayments don’t have to be the full par value of the bond.

Both prepayment options and call options shorten the effective duration of a bond; hence, lower interest rate risk than an equal-maturity, option-free bond.