In short he used a rub brick, some Salba sauce and some wax to make the ledge as nice as possible for grinding & sliding.
If you are looking for directions on how to do it -
Hopefully this curb/ledge has never been waxed. If it has, abandon it and find a clean section and check it for cracks and expansion joints so you know whether or not you need Quiksteel to fill them in.
Go to the hardware store and pick up
rub brick (or 2 if you have helpful, willing skate friends)
medium-large phillips screwdriver (to tighten rub brick handle when it loosens)
brush or broom (cheapest one with removable handle, leave handle at home/in car)
duct tape (cheapest they have is fine)
clear spray lacquer (Enamel also works, just dries slower. 1 can will do approximately 10-12' of single sided curb depending on paint thickness and wind conditions)
Steelstik (if necessary - this is a 2 part epoxy made by JB Weld that you mix in your hand, roll into thin rope and stuff neatly into crack)
rubber gloves (only if using Steelstik. cheap dishwashing styles work well, these will be thrown away)
First you want to rub brick the shit out of it to make it real nice. When using the rub brick you want to lean on it a bit to apply pressure and really grind those ridges flat. You will work up some dust and need to brush it off your work surface occasionally or it will greatly reduce your effectiveness. Keep rubbing until the surface is about smooth as a countertop across the top and about 3 inches down the front edges.
Next you will want to sweep the entire surface really well. Step back and look at the length of curb/ledge you have smoothed. If your desired distance of curb contained any major cracks or expansion joints, you can now carefully fill them in with the Steelstik. If you think you would like to extend your sweet ass smoothed curb later, take the duct tape and tape over the last few inches of smooth concrete on the ends. This will allow you to paint and shred the section of curb/ledge you have ground smooth now but make it easier to tie in to your next rub bricking session as it gives you a nice, clean, paint-free and pre-done area after you paint and remove it. If you don't do this, it will be a bitch to grind through the clear lacquer with your rub brick next time.
Then shake the daylights out of your rattle can clear lacquer and get ready to spray. Hold the can about 10-12 inches away if no breeze at all, a bit less if windy and spray the smoothed portion of the curb/ledge with a light coat from tape edge to tape edge (enough to barely look damp, not enough to look wet, yet) and let it sit for 5 minutes. Spray another medium coat and make it look damp and let it sit for about 15 minutes. Spray one final medium-thick coat and it should look uniformly wet when done. Peel the tape off and throw it away in the empty plastic bag you brought in your pack.
Next you pack up quickly and get the hell out of there. Go home so you don't get busted there waiting for it to dry by some cop who was called by a neighbor about the loud noise or the gluesniffer they saw feverishly rubbing the shit out of the sidewalk. Get cleaned up, change clothes, have a snack, make a bm, play some THPS2, listen to some KMK and take bong rips, watch Chomp On This VHS and roll out.
Finally you and everyone else from now until they end of time can shred the shit out of the perfect curb/ledge that YOU created. Enjoy it and always be on the lookout for the next great curb or ledge spot that you can help make skatable for everyone.
PS: Try to bring a friend or two to rotate with you between look out and grinding duty and remember to stay as inconspicuous as one can while being loud as fuck grinding down concrete by hand. Any passersby will wonder why you're spray painting invisible graffiti on some random curb with your strange friends so I treat the act as such and try not to let them see. You can also add a bit of wax if you want it super slick.