I think they made the distinction with the question “what if the deadline slips?”

If you don’t finish your costume in time for a con, you miss it. No big deal.

If you miss delivering a costume for client for a con, very big deal.

Well, if you're planning a costume for a whole year and then don't finish in time, it's a huge deal, at least for cosplayers that I know. Many of them actually postpone the real-life work before big events in order to finish the costumes and prepare everything. For them it's more of an obsession than just pastime, but still it's a hobby.

Obsession over hobby makes it not a hobby anymore. Been there, done that.

If you keep putting yourself in a position where you have aggressive hard deadlines.. very big deal.

Yeah, social hobbies often don't fit the “no deadlines” definition.

If there are no consequences for missing a deadline, why even call it that?

There are obvious consequences within the scope of the hobby itself, even if there aren't to your overall life. If you don't get the costume done in time for the convention, then you either miss the convention or go without the costume. Those are the consequences for missing that particular deadline.