Can you clarify "coding it all together" part. I have no idea what you meant.
Of course for more than trivial built-in behaviours you'll be writing code, but for a UI prototype, especially if it's just CRUD forms etc., you can get it built in a few minutes with no code. Drop a database connection component and set the connection properties, and you'll get live data and properly typed and bound controls on the form too.
Compilation takes fractions of a second, so that's not an impediment to design iteration - but you don't need to compile or run to see what it looks like since it's effectively a WYSIWYG form designer.
[Edit: hit reply too soon] Tk model of UI programming does not promote this that much, because causing stuff to happen by writing code is the default way. Also Tk-style approach of writing GUI definitions in code and letting the toolkit to deal with details like positioning (in fact, using HTML for UI is mostly similar) is more productive.
I really wish HTML was as easy to design with as the VCL designer. Trying to e.g. create vertically centered children is far, far easier in the VCL than in HTML + CSS.
Rather, it'll be limited to Windows, macOS, iOS and Android. :)
Very limited ones. And ones clients will have a shock when seeing. And still coding it all together.
With Delphi you can drag and drop great looking (and more full featured) prototypes in zero time.
It will be limited to Windows, though, I'll give you that.