Good question and two things stand out for me:
1 Sowing parsnips in February does not necessarily mean a bigger harvest, for example older parsnips suffer more canker, though mostly on heavy soils, less on light ones.
2 For the small benefit of pre-covering with fleece, I am unconvinced that its worthwhile. Sure lay it over sowings, and they are germinating for a long time, so the warming effect coincides with that. I favour sowing mid March, roughly (vis weather, moon) and laying fleece over sowings, not before.
