You can certainly sow seeds however you want, but Winter Sowing is not direct sowing. It's a superior method because the containers protect the seeds from critters, insects, washouts, desiccation and windblown weed seeds.

I would suggest that for comparison, you do try both direct sowing and winter sowing the same amount of seeds and at the end of the growing season you campare the results.

Both methods will create hardy seedlings because germination is entirely outdoors, but with WS, because of the protective containers you will have a tremendous amount more of seedlings because more seeds survive the winter.

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If you applied a winter mulch and had seedlings in Spring, you may have been lucky. One of the purposes of mulching is to suppress germination - of weed seeds, but same is true of any seed. Most seeds buried under a layer of mulch are not going to germinate.