Comments like you report drive me nuts!
They are so vague, you are right to wonder if there is truth in them and 99% of the time there is not. And if there were, no problem.
It all depends what seeds were in the horses’ hay, some will still be in the composted manure.
Then it it is so, so simple to either pull out any germinating weed seedlings, or hoe them off, or kill them with a rake on a dry afternoon etc.
Also while spreading manure, break it to small pieces with a fork and pull out any roots of perennial weeds.
