It's broadly worded precisely so that people can construe it as they like. If the SoI said "labor-based property," it would exclude most libertarians. If it said "both land- and labor-based property," it would exclude Georgists. As it is, I think it can include both right- and left-libertarians, within reason. (Some self-described left-libertarians think that all property is theft & that it is acceptable to use violence to socialize the means of production in an "anarchist" society. They are excluded from the movement.)