They also stated that there was no significant velocity difference from 26" down to 20" for the given load.

From the article:
"During the development of the Tango 51, Tac Ops took a standard 26-inch barrel and cut it down to 18 inches in one-inch increments. Between 10 to 20 rounds were fired at each invrement. They found that a 20-inch barrel provides for a complete propellant burn and no velocity loss when using Federal Match 168-grain BTHP, a cartridge that has become something of a law enforcement standard. Going to an 18-inch barrel only resulted in a loss of 32 feet per second (fps).

The main difference I see is that the barrels they were testing were heavier contours unlike the S&W barrel's last 6" or so.

Regarding the load tested, it was used because it is one of the more commonly used factory loads for precision shooters. While it may not hold true for every possible load, one might draw the conclusion that loads using a powder with a similar burn rate will react similarly.