You are right and I say in the post that I think “the West Riding compost is a little rich and/or dense for seeds of lettuce, spinach and beetroot. But cabbage, dill and coriander have come up fine”.
Funnily enough my later sowings of spinach, beetroot & lettuce in the same W Riding compost came up fine. So it may have been more to do with drainage as early sowings sit for longer in high levels of moisture. My trays were over a humid hotbed and that can be tricky in February.
Hence adding vermiculite, sand etc is a good plan, for sowing only.
