I don't think that is quite what is meant by filling the container or taking on the shape as mentioned earlier.
Take a container.
The difference in behaviour as regards 'filling' between a gas and a liquid is simple here.
For a gas there is no part of the volume that is not available to the gas molecules to occupy.
Yes there is only a finite probability of finding a gas molecule in any given region at any given time, and yes the distribution will be uneven and vary with time, but unlike a liquid, there is no boundary or surface.
A liquid on the other hand has a surface. All the liquid molecules occupy space on one side of the surface only.
Yes there may be vapour molecules escaped from the surface in the rest of the container, but these are no longer liquid.