Left-libertarianism is a subset of socialism. Socialism is a wide umbrella term for multiple economic systems that put the people doing the work in charge of the workplace. It has nothing directly to do with a 'harmonious society'
What exactly is "socialist" about Stallman, aside from him not being a US Republican and disliking Big Tech?
For example, a radically socialist policy would be that "social ownership" should extend to some amount of personal information, to increase accountability, cohesion, trust, etc. But Stallman would vehemently oppose this.