My first guess would be that the tire was old. It looks like a new wheel with the green sticker on it and the tread seems to look like new. Maybe it was a spare that was REALLY old and never used before? I doubt speed rating had anything to do with this failure. Most of us never reach the speeds the tires are rated for.
The section laying on the ground is bald, but the tread section attached to the carcass on the wheel has a lot of tread. This looks like he got a piece of semi-truck retread hung up in his wheel well and it took out his tire.
That looks like a Ford OEM spare combination. It does look like part of a tire from another vehicle is hung up in there.
Hard to tell from the picture. If that’s the original spare, it was on borrowed time anyway.
To answer the question: What makes you think that isn't an H rated tire? It appears to have a circumferential cap ply.
Background: I used to analyze failed tires for a living. The tire manufacturer I worked for depended on my analyses to pay or deny claims of property damage (up to $5,000 per incident) Needless to say, my work was scrutinized. (No, we had someone else deal with lawsuits!)
Some interesting tidbits: The final production for Mercury Grand Marquis's was Jan, 2011. If the photo was recently taken, that would make the vehicle 10 years old.
Note the yellow (green) sticker on the wheel. That's the low point of the wheel used for match mounting. It's about 180° away from the valve stem - meaning that these wheels did NOT use the valve hole to indicate the match mounting point. To my knowledge only Chrysler has recently used the valve hole for match mounting - meaning that for other vehicles matching the dots on tires to the valve hole in the wheel doesn't do anything useful.
Note also that some of the lug nuts have lost their chrome covering.
There appears to be some damage to the lower fender near the rocker panel, but the rest of the fender doesn't appear to be damaged. That probably means the failure occurred at moderate speed - enough speed for the centrifugal forces to open up the tread flap, but not enough to damage the fender (except for the inside the fenderwell.)
Oh and the tire failed due to a tread separation - belt leaving belt variety - and that resulted in a tread flap being created. That flap likely hit the rocker panel and pulled the tread off the rest of the tire, but since the separation hadn't grown much, the flap only contained a small portion of the top steel belt and cap ply. The bulk of the steel belts were still attached to the rest of the tire.
The puzzling thing is that the photo doesn't show any frayed fabric at the edge of the tread flap. Usually a cap ply doesn't break cleanly. If there is no cap ply, why are there circumferential markings further down the tread flap?