I could see it being done with a brush more credibly, yeah. I just don't think the left edge of the stroke in ylf can be drawn straightly with a normal quill or pen without a lot of awkwardness or a custom tool. And yes, fimfa's arm is perfectly drawable with the edge of a tool, it's just a stylistic anomaly compared to the rest of the script, as you have a thin line at the same angle as the thickest parts of every other letter.