This is a great observation and a good thing to keep in mind for worldle (would have been helpful for the South Atlantic Island the other day).

One of the weirder concepts to wrap your head around in sedimentology is relative sea level. Sea level at any location can change due to local effects, like heavy glaciers loading the crust, actually bowing it down, or for global effects, like changes in global climate or rate of sea floor spreading. It sounds pretty straightforward, but trying to sort out local vs global effects was really hard and took decades.

On a stable coastline, one where relative sea level has been more or less constant for a long time, sediments fill the basin in the water, and the coastline starts to advance out into where the water was before. This is what deltas do. When you look really closely, they are complicated and jagged too, but not on a regional scale.

On a coastline where there is rapidly changing sea level, particularly rapid rises in sea level, all the smoothed coastline will be underwater, and the new coastline will be jagged.

In Earth’s current state, the poles have experienced rapid, recent (50,000 yrs) changes in sea level due to glacial cycles, AND, the erosional pattern of alpine glacier leads to deep, steep valleys (glacier=sediment bulldozer, river = central conveyer belt), so the squiggliness of the coastline will be even more.