I can’t help with advice on this seeder. For small seeds such as carrots I find it best to keep my finest compost for where they are to be sown, spread not too thick (an inch at most on the surface). Forking has never been a problem. The advice on not manuring ground for root crops assumes that you are digging it in – it is the incorporated manure or compost which leads to fangs and forks.
I have seen this on my dig / no dig experiment where surface composted parsnips are all straight, compared to those growing in a dug bed with compost dug in, which often fork.