I think it's piping gel...the Wilton kind you can buy in the tub. They could have used black BC for the outline though. Let it set up and then go over it all with red piping gel. There are some instructions here on using piping gel to make designs on cakes...
Use Piping Gel to Make Complex Designs on Your Cakes and Cookies:
Tint piping gel (Use gel or paste colors) with either paste or gel colors so it is visible against the icing for a cake. For a cookie, tint Piping gel so the design will contrast with it.
Put it in a pastry bag fitted with a #1 tip or a plastic squeeze bottle (my favorite).
Draw a pattern on a piece of wax paper using an indelible marker.
Turn the wax paper over so the ink in on the backside. Spray backside with a light coating of vegetable oil spray. You can lay the wax paper on top of a design and simply trace with Piping gel!
CAREFULLY place side with gel design on top of cake or cookie its best if you have let the frosting harden for a few minutes if decorating a cake.
Carefully press the gel with your fingers gently -- and then lift the wax paper off. The design will have transferred to the cake or cookie. You should be able to get two or three transfers from each sheet of wax paper.
Now use a slightly larger decorating tip (or squeeze bottle) with regular frosting and go over the gel outline then fill with colored Piping gel or icing. (Use gel or paste colors).