They probably do believe in it, if rephrased slightly. The term "central planning" is now indelibly associated with the USSR and failure so nobody supports it anymore, but take concepts that are very similar and all sorts of people including self-declared socialists will line up in praise. For example:
- Health authorities should run society and the economy during a pandemic.
- Government policy should be made by experts, not politicians.
- The EU's constant attempts to create national tech champions via grants.
- "Reparations"
- Net Zero
These are all examples of plans for top-down planning of society and the economy by supposedly rational and disinterested intellectuals. We don't use the exact term central planning for these things, but they have the same sort of effect.
Yes, on the internet you can find people who claim to be anarcho-communists, just like you can find people claiming to be a lot of extremely niche things. Good luck finding them in the world of real offline politics!
However, if you'll notice, here on HN a lot of people calling themselves socialists don't believe in the idea of central planning, so maybe there's more to it that socialists are just a sort of person.