Ijsselmeer is a different kind of lake; not created by digging out peat, but a (violent) sea inlet that was closed off.

Hmm yes, 'everybody' in the NL is aware of that.

In the latter case, they sometimes left those strips of land to transport the fuel.

Not really. The turf blocks would be stacked in piles & left to dry for a while. But then often transported by boat (through the newly created channels). The leftover strips of land could be used to have men, or a horse, pull such boats on a rope.