The warheads will be going mach 15 at top speed. What they are doing I think is trading off the MIRVs for an advanced evasion system designed to get one warhead (per missile) through an ABM defense. This could be overcome with SBL (space based lasers) or nuclear ABMs. Something like Brilliant Pebbles with an SBL component would be impervious to their "impervious missile."
The faster a missile travels through the air the greater the pressure upon the missile by the atmosphere. If it's doing Mach 15 then slight damage to it could destroy it. The ABL can punch a basketball sized hole in a regular missile. I doubt this system could survive that.