Letīs have a look at it this way:
A plane at a given altitude (air density) and low Mach number has a Cl which depends on angle of attack, and that Cl has a maximum. That is where the plane stalls: any increase of AoA causes drop of lift. And decrease of AoA also causes decrease of lift. A plane flies at stall speed when its total weight is equal to the local maximum of lift for that air density and true airspeed. Right?
Note that at stall, Cd is substantial. A plane can have lower Cd, and bigger L/D, by flying faster. It is perfectly possible for a plane to have no thrust at all, or to have thrust which, at stall, is less than the drag at stall.
Now, with changes of Mach number the Cl will change. Generally, in nearsonic region, it decreases, so that supersonic Clmax is lower than subsonic Clmax. But not zero.
In nearsonic region the Clmax decreases with Mach number so fast that the product Clmax times V squared has a local maximum - increase of airspeed decreases the maximum lift over the given airspeed.
This is a local maximum - in supersonic range the Clmax may decrease further, but the lift shall increase with V squared, without bound.
The local maximum of lift equalling the total weight defines the coffin corner. Right?
Now, the available thrust changes depending on air density and true airspeed. Since the coffin corner is at stall (a high drag condition anyway), it makes sense that the engines may not provide the thrust necessary to reach and sustain coffin corner altitudes...