Good answers here and the question should read:
Is compost made from horse manure alone ok to plant in?
The manure word is highly confusing as people use it for both fresh and decomposed.
It’s absolutely fine to sow and plant into the latter and as you suspect, there is no need for complicated layers and mixes of different composts/soil/vermiculite/peat etc which one sees recommended, often in bizarrely precise amounts!
Sometimes with old but rather lumpy manure, a sack of potting compost on top gives a smoother layer for planting, and it’s easier to look after small seedlings.
