Why did the Mercure use cascade-type reversers?
Did Dassault have some magic trick up their britches

It was an era of innovations. Have you seen the Vickers VC10 thrust reversers? The clamshell / target doors can be made internal within the nacelle. Literature is lacking on the Mercure, but there is a nice hint from a cutaway drawing:


Source: Air Enthusiast magazine, via aviadejavu.ru

The highlighted item relates to the exhaust cone, and it's labeled &quot;Thrust-reverser cone.&quot; I will use a different patent (for now) for illustrative purposes only:



Above shows a nacelle that cascades, revealing vane panels to direct the gases forward, and blocker doors that pivot radially inwards.
The Mercure's thrust reversers were supplied by Snecma, now Safran, and indeed I found a Safran patent for a thrust-reversing cone of similar embodiment that was filed in 1974 (the year of Mercure's entry into service). It does not have cool drawings, but the text is clear:

The jet propulsion nozzle 7 is here of the needle or plug type, or in other words provided with a central body 30 [i.e. cone], and can be closed off completely by means of flaps (not shown) which seat (in the closed position) on said plug or central body 30. [emphasis mine]

Jumelle, Louis Francois. &quot;Jet engine nacelle with drag augmenter auxiliary for thrust-reverser system.&quot; U.S. Patent No. 3,941,313. 2 Mar. 1976.