As oppose to girth hitching like what? Is this not a girth hitch? Also, any sort of bend, hitch or knot in a sling, or even a climbing rope, will have reduced breaking strength. I think Nick's statement is an overgeneralization about a girth hitch's breaking strength in a static environment and for what it's worth I'd bet to say that in this situation nylon will have the higher breaking strength over a dyneema sling.
The reason this is "cinching" girth hitch sketchier than a standard girth hitch is because of the force multiplication created by "hard" bend in sling acting like a pulley. Whereas a less aggressive cinching in action would create less force multiplication.
In some situations the loss of strength is worth the stability. This doesn't look like one of those situations.
EDIT to add: It looks like (since I'm assuming right is down) switching which end passes through the other would make this a stronger configuration. Ultimately it probably isn't that big of deal anyway.
If one really wanted to be a stickler for detail. Technically speaking the cinching hitch doesn't actually weaken the sling any more than a regular girth hitch - it does however increase the force on the "pulley" end of the sling. In a frictionless world it would create a 2-1 pulley system thereby making the effective strength of the sling half it's "normal" girth-hitched strength.