Hello! You are correct I that I just drew it. Basically, I just take a Photoshop canvas sized at 2,500 x 2,500 pixels (the largest possible size for a brush) and I draw whatever I want my brush to be. If you only want it to be a stamp, then you draw the brush in black on a white background and then select Edit > Define Brush Preset. And then you have a stamp!
The newest version of my watermark can be seen here.