Expression in %if can be any valid macro expression that resolves to an integer. If expression is evaluated to zero, then it is false and the %then text is not processed. If it resoles to any integer other than zero, then the expression is true, and the %then text is processed. If it resolves to null or to any noninteger value, an error message is issued.
The SAS macro code is using a technique to force a TRUE condition test result. A value of 1 is TRUE and a value of 0 is false. You can see this processing decision during SAS code compilation if you turn on: