Perhaps it should be something along the lines of "final price exactly as quoted including tip, nothing more" the key being that they aren't one of those services (like so many others) that advertise one price and charge you another.
"no hidden fees" is a recognizable meme though, and being pedantically correct is perhaps – and this is a pedantic concern – not as valuable as the phycological grab of that phrase.
Unless they're working out something with the provider where the third party will not expect a tip, which would be a miracle.
Edit: I stand corrected. Someone quotes the site as having:
> It's completely free to chat with Magic. When you order something, we'll let you know the total price so you can confim it before you are billed. There are no hidden fees, and tip is included.