The point was that when something is large enough to be “an afternoon”, it's probably more work than you're expecting and you haven't yet discovered important details. If there's something which does what you need, it's far more likely that _other people_ have invested time sanding off the rough edges which you have yet to discover.

If it's not hard to use that library you're probably better off unless it's a problem you understand very well and will see a real advantage to tackling differently. For example, if you use a library and don't like it that experience will still be useful for having clarified what exactly it is that you want to do and the rough size of what you're taking on.