Welcome aboard! You are misreading the statement. They didn’t stop custom parts (and with sufficient knowledge, you can create a part from scratch), they are suggesting that you start from an existing part because the required file format is complex and poorly documented (and the Parts editor is still a work in progress and needs a properly formatted base file to work from, and has limitations because it is not yet complete.) That said learning to make parts isn’t particularly easy, but a number of us will help if you run in to problems. These two tutorials on parts making apply to the current version of Fritzing (most of the rest are for older versions of the software), but asking here is still probably the best way to get answers until we get better documentation together.