Since many illnesses can be transmitted by contaminated hands, the idea is plausible, but it's a good example of the media misinterpreting a study and misleading naïve readers..
They measured the surface area of open hands and fists in 10 subjects. Not surprisingly surface area of an open hand was significantly greater than that of the fist—30.206 vs. 7.867 sq. in. respectively, p < 0.00001.
They also measured the contact time of handshakes and fist bumps. The handshake took 2.7 times longer than that of the fist bump (0.75 vs 0.28 seconds). [No statistical analysis provided]