Except they don't, so the whole blog post should be taken with a grain of salt.
I just signed a document a few days ago (on Sept 28th). I didn't like the default font for signature they offered, so I picked some other, script type of font. Everything was fine until I saved it. And suddenly, the signature switched to some completely different serif font.
The document wasn't really that important to me, so I let it go like that, but doesn't look like something I would trust currently.
EDIT: HelloSign people are obviously reading this. Instead of down-voting me, the proper approach would be to ask for details and fix your bugs. For example, I just tried another document on Windows and it worked properly. For the problematic one, I was using Firefox on macOS, so maybe you should look into that. I can also send you the document ID hash if you ask nice.