Fountain pens don't leave indentations on the paper, but there are often differences in ink density in parts of the letters or punctuation. This is much less noticeable w/ a fountain pen than it is with a calligraphic or dip pen, but still can be noticeable. [1]
Since these differences in ink density are caused by variations in nib speed and pressure while writing, I think that's what they were referring to by "pressure marks". For me, it seems to depend both on the pen and the paper I'm using. It's possible that what you use doesn't show much of this effect at all. (I also use too much pressure with a fountain pen, which likely deposits more ink and hides effects like this.)
When I'm not doing physics homework in Mathematica, I do it with fountain pen. It's rather enjoyable.