One could argue that, and it's something that makes sense on the surface. But anarchism at its core is about the destruction of what's seen as unjust hierarchies, with decentralization a part of that. Anarchism is in the end about the individual: it places individual freedom over collective freedom, as can be seen , for example, at their dismissal of every socialist project due to apparent "authoritarianism" over structures in place (i.e. "hierarchy") to maintain collective freedom of the masses.
Each lemmy instance has a hierarchy and rules put in place by that hierarchy that limit individual freedoms. All users aren't congregated together to jointly come up with instance or community rules, etc. Even in the most "open" forum we have, the internet, hierarchies are still in place to maintain a smoothly functioning community.
The argument to make is that libre software is more communist as every person has the means of production in hand (a computer) and strong copy-left licenses limit individual freedom (you can't take the source code and use it for profit without making it open) for collective freedom (the code is always open source for people to use/audit).