I'd assume they're using InVision, it says it's for an InVision project in the description..
>Here is another ui design I did for the secret InVision project. Stay tuned for more stuff.
I've never used InVision but Pixate (mentioned by /u/LobotomizedGeek) is free and fairly simple and could pull off something very similar.
It's just boilerplate legalese. I picked one with a similar name to your username: http://www.pixate.com/terms/. Ctrl+F "virus".
If you google around, almost every ToS includes some variant of that line. Some mention "viruses and worms", some mention malware, some omit it, some use it verbatim.
Is it possible that an attacker could distribute a modified version of their software to include malware? Yes. So they add the provision that there's no guarantee that someone hasn't added a virus. You have to remember, this is what they're going to use to defend against a lawsuit. If it can happen, they'll probably mention it.
These are specific to UI:
http://www.relativewave.com/form/
Framer pairs up well with sketch, but I haven't had a chance to use it yet. It requires some pretty basic understanding of code. Pixate is pretty good, but you need to recreate your design in their app and its a bit cumbersome. Form seems really powerful, but also really complex to learn.
Pixate is great free tool that leverages the iOS simulator and creates lush animations. Pretty sure it will get you what you want. Low to medium learning curve with nice documentation. http://www.pixate.com grab the native app for best performance and results. Here are the demos http://www.pixate.com/education/demos/
WYSIWYG just aren't advanced enough to produce modern optimized code for long-term functional use.
To have 100% control of your work, there's no alternative for working straight with the code, for now at least.
As a prototyping system, there are much better purpose-built programs out there, depending on what you're concerned with: Sketch, Pixate, Google Web Developer. But a WYSIWYG isn't built with prototyping in mind, so its design hinders efficiency in that respect.
As far as I undertood, you don't really need a working app (in terms of actual implementation). You culd try pixate. I didin't try it myself but it seems pretty neat because you can actually "use" it on the device. Foremost, it is free as it was aquired by Google.
I had some fun with pixate freestyle for styling within Xcode. I ended up not using it but it did work pretty well and I think would be more valuable if I wasn't using storyboards so much.
I usually use interface builder for making interfaces because most UI customisation could be done directly from there (99% of tasks - the only example that I remember now is setting cornerRadius or other "lower level" stuff). Even if you make a subclass you can still create the view using a nib file(or more).
Another thing i found useful was using Pixate (http://www.pixate.com/) Pixate allows you to style your app using CSS - the cool thing about it is that your nib file could be left without customisation and just work on the layout in interface builder. Then you can customise the app in real time by changing the CSS file and see the results immediately. (old youtube video of this feature http://www.youtube.com/watch?v=rjNrNIyEL_c&feature=youtu.be&t=2m38s)
> http://www.pixate.com/education/
Interesting. At first I was like, "why the hell would I use this instead of storyboards in xCode". But after playing with it, I could see some benefits especially for non-tech people. I'll play with it some more, but not sold just yet.
this video really is a good one for going over basic animation
Also there is a new program out there called pixate which is very easy to get to know and I've been spearheading some mockups per my companies request.